diff options
| -rw-r--r-- | drivers/lguest/core.c | 273 | ||||
| -rw-r--r-- | drivers/lguest/hypercalls.c | 118 | ||||
| -rw-r--r-- | drivers/lguest/interrupts_and_traps.c | 176 | ||||
| -rw-r--r-- | drivers/lguest/lg.h | 19 | ||||
| -rw-r--r-- | drivers/lguest/page_tables.c | 314 | ||||
| -rw-r--r-- | drivers/lguest/segments.c | 109 |
6 files changed, 924 insertions, 85 deletions
diff --git a/drivers/lguest/core.c b/drivers/lguest/core.c index 1eb05f9..c0f50b4 100644 --- a/drivers/lguest/core.c +++ b/drivers/lguest/core.c @@ -64,11 +64,33 @@ static struct lguest_pages *lguest_pages(unsigned int cpu) (SWITCHER_ADDR + SHARED_SWITCHER_PAGES*PAGE_SIZE))[cpu]); } +/*H:010 We need to set up the Switcher at a high virtual address. Remember the + * Switcher is a few hundred bytes of assembler code which actually changes the + * CPU to run the Guest, and then changes back to the Host when a trap or + * interrupt happens. + * + * The Switcher code must be at the same virtual address in the Guest as the + * Host since it will be running as the switchover occurs. + * + * Trying to map memory at a particular address is an unusual thing to do, so + * it's not a simple one-liner. We also set up the per-cpu parts of the + * Switcher here. + */ static __init int map_switcher(void) { int i, err; struct page **pagep; + /* + * Map the Switcher in to high memory. + * + * It turns out that if we choose the address 0xFFC00000 (4MB under the + * top virtual address), it makes setting up the page tables really + * easy. + */ + + /* We allocate an array of "struct page"s. map_vm_area() wants the + * pages in this form, rather than just an array of pointers. */ switcher_page = kmalloc(sizeof(switcher_page[0])*TOTAL_SWITCHER_PAGES, GFP_KERNEL); if (!switcher_page) { @@ -76,6 +98,8 @@ static __init int map_switcher(void) goto out; } + /* Now we actually allocate the pages. The Guest will see these pages, + * so we make sure they're zeroed. */ for (i = 0; i < TOTAL_SWITCHER_PAGES; i++) { unsigned long addr = get_zeroed_page(GFP_KERNEL); if (!addr) { @@ -85,6 +109,9 @@ static __init int map_switcher(void) switcher_page[i] = virt_to_page(addr); } + /* Now we reserve the "virtual memory area" we want: 0xFFC00000 + * (SWITCHER_ADDR). We might not get it in theory, but in practice + * it's worked so far. */ switcher_vma = __get_vm_area(TOTAL_SWITCHER_PAGES * PAGE_SIZE, VM_ALLOC, SWITCHER_ADDR, VMALLOC_END); if (!switcher_vma) { @@ -93,49 +120,105 @@ static __init int map_switcher(void) goto free_pages; } + /* This code actually sets up the pages we've allocated to appear at + * SWITCHER_ADDR. map_vm_area() takes the vma we allocated above, the + * kind of pages we're mapping (kernel pages), and a pointer to our + * array of struct pages. It increments that pointer, but we don't + * care. */ pagep = switcher_page; err = map_vm_area(switcher_vma, PAGE_KERNEL, &pagep); if (err) { printk("lguest: map_vm_area failed: %i\n", err); goto free_vma; } + + /* Now the switcher is mapped at the right address, we can't fail! + * Copy in the compiled-in Switcher code (from switcher.S). */ memcpy(switcher_vma->addr, start_switcher_text, end_switcher_text - start_switcher_text); - /* Fix up IDT entries to point into copied text. */ + /* Most of the switcher.S doesn't care that it's been moved; on Intel, + * jumps are relative, and it doesn't access any references to external + * code or data. + * + * The only exception is the interrupt handlers in switcher.S: their + * addresses are placed in a table (default_idt_entries), so we need to + * update the table with the new addresses. switcher_offset() is a + * convenience function which returns the distance between the builtin + * switcher code and the high-mapped copy we just made. */ for (i = 0; i < IDT_ENTRIES; i++) default_idt_entries[i] += switcher_offset(); + /* + * Set up the Switcher's per-cpu areas. + * + * Each CPU gets two pages of its own within the high-mapped region + * (aka. "struct lguest_pages"). Much of this can be initialized now, + * but some depends on what Guest we are running (which is set up in + * copy_in_guest_info()). + */ for_each_possible_cpu(i) { + /* lguest_pages() returns this CPU's two pages. */ struct lguest_pages *pages = lguest_pages(i); + /* This is a convenience pointer to make the code fit one + * statement to a line. */ struct lguest_ro_state *state = &pages->state; - /* These fields are static: rest done in copy_in_guest_info */ + /* The Global Descriptor Table: the Host has a different one + * for each CPU. We keep a descriptor for the GDT which says + * where it is and how big it is (the size is actually the last + * byte, not the size, hence the "-1"). */ state->host_gdt_desc.size = GDT_SIZE-1; state->host_gdt_desc.address = (long)get_cpu_gdt_table(i); + + /* All CPUs on the Host use the same Interrupt Descriptor + * Table, so we just use store_idt(), which gets this CPU's IDT + * descriptor. */ store_idt(&state->host_idt_desc); + + /* The descriptors for the Guest's GDT and IDT can be filled + * out now, too. We copy the GDT & IDT into ->guest_gdt and + * ->guest_idt before actually running the Guest. */ state->guest_idt_desc.size = sizeof(state->guest_idt)-1; state->guest_idt_desc.address = (long)&state->guest_idt; state->guest_gdt_desc.size = sizeof(state->guest_gdt)-1; state->guest_gdt_desc.address = (long)&state->guest_gdt; + + /* We know where we want the stack to be when the Guest enters + * the switcher: in pages->regs. The stack grows upwards, so + * we start it at the end of that structure. */ state->guest_tss.esp0 = (long)(&pages->regs + 1); + /* And this is the GDT entry to use for the stack: we keep a + * couple of special LGUEST entries. */ state->guest_tss.ss0 = LGUEST_DS; - /* No I/O for you! */ + + /* x86 can have a finegrained bitmap which indicates what I/O + * ports the process can use. We set it to the end of our + * structure, meaning "none". */ state->guest_tss.io_bitmap_base = sizeof(state->guest_tss); + + /* Some GDT entries are the same across all Guests, so we can + * set them up now. */ setup_default_gdt_entries(state); + /* Most IDT entries are the same for all Guests, too.*/ setup_default_idt_entries(state, default_idt_entries); - /* Setup LGUEST segments on all cpus */ + /* The Host needs to be able to use the LGUEST segments on this + * CPU, too, so put them in the Host GDT. */ get_cpu_gdt_table(i)[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT; get_cpu_gdt_table(i)[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT; } - /* Initialize entry point into switcher. */ + /* In the Switcher, we want the %cs segment register to use the + * LGUEST_CS GDT entry: we've put that in the Host and Guest GDTs, so + * it will be undisturbed when we switch. To change %cs and jump we + * need this structure to feed to Intel's "lcall" instruction. */ lguest_entry.offset = (long)switch_to_guest + switcher_offset(); lguest_entry.segment = LGUEST_CS; printk(KERN_INFO "lguest: mapped switcher at %p\n", switcher_vma->addr); + /* And we succeeded... */ return 0; free_vma: @@ -149,35 +232,58 @@ free_some_pages: out: return err; } +/*:*/ +/* Cleaning up the mapping when the module is unloaded is almost... + * too easy. */ static void unmap_switcher(void) { unsigned int i; + /* vunmap() undoes *both* map_vm_area() and __get_vm_area(). */ vunmap(switcher_vma->addr); + /* Now we just need to free the pages we copied the switcher into */ for (i = 0; i < TOTAL_SWITCHER_PAGES; i++) __free_pages(switcher_page[i], 0); } -/* IN/OUT insns: enough to get us past boot-time probing. */ +/*H:130 Our Guest is usually so well behaved; it never tries to do things it + * isn't allowed to. Unfortunately, "struct paravirt_ops" isn't quite + * complete, because it doesn't contain replacements for the Intel I/O + * instructions. As a result, the Guest sometimes fumbles across one during + * the boot process as it probes for various things which are usually attached + * to a PC. + * + * When the Guest uses one of these instructions, we get trap #13 (General + * Protection Fault) and come here. We see if it's one of those troublesome + * instructions and skip over it. We return true if we did. */ static int emulate_insn(struct lguest *lg) { u8 insn; unsigned int insnlen = 0, in = 0, shift = 0; + /* The eip contains the *virtual* address of the Guest's instruction: + * guest_pa just subtracts the Guest's page_offset. */ unsigned long physaddr = guest_pa(lg, lg->regs->eip); - /* This only works for addresses in linear mapping... */ + /* The guest_pa() function only works for Guest kernel addresses, but + * that's all we're trying to do anyway. */ if (lg->regs->eip < lg->page_offset) return 0; + + /* Decoding x86 instructions is icky. */ lgread(lg, &insn, physaddr, 1); - /* Operand size prefix means it's actually for ax. */ + /* 0x66 is an "operand prefix". It means it's using the upper 16 bits + of the eax register. */ if (insn == 0x66) { shift = 16; + /* The instruction is 1 byte so far, read the next byte. */ insnlen = 1; lgread(lg, &insn, physaddr + insnlen, 1); } + /* We can ignore the lower bit for the moment and decode the 4 opcodes + * we need to emulate. */ switch (insn & 0xFE) { case 0xE4: /* in <next byte>,%al */ insnlen += 2; @@ -194,9 +300,13 @@ static int emulate_insn(struct lguest *lg) insnlen += 1; break; default: + /* OK, we don't know what this is, can't emulate. */ return 0; } + /* If it was an "IN" instruction, they expect the result to be read + * into %eax, so we change %eax. We always return all-ones, which + * traditionally means "there's nothing there". */ if (in) { /* Lower bit tells is whether it's a 16 or 32 bit access */ if (insn & 0x1) @@ -204,9 +314,12 @@ static int emulate_insn(struct lguest *lg) else lg->regs->eax |= (0xFFFF << shift); } + /* Finally, we've "done" the instruction, so move past it. */ lg->regs->eip += insnlen; + /* Success! */ return 1; } +/*:*/ /*L:305 * Dealing With Guest Memory. @@ -321,13 +434,24 @@ static void run_guest_once(struct lguest *lg, struct lguest_pages *pages) : "memory", "%edx", "%ecx", "%edi", "%esi"); } +/*H:030 Let's jump straight to the the main loop which runs the Guest. + * Remember, this is called by the Launcher reading /dev/lguest, and we keep + * going around and around until something interesting happens. */ int run_guest(struct lguest *lg, unsigned long __user *user) { + /* We stop running once the Guest is dead. */ while (!lg->dead) { + /* We need to initialize this, otherwise gcc complains. It's + * not (yet) clever enough to see that it's initialized when we + * need it. */ unsigned int cr2 = 0; /* Damn gcc */ - /* Hypercalls first: we might have been out to userspace */ + /* First we run any hypercalls the Guest wants done: either in + * the hypercall ring in "struct lguest_data", or directly by + * using int 31 (LGUEST_TRAP_ENTRY). */ do_hypercalls(lg); + /* It's possible the Guest did a SEND_DMA hypercall to the + * Launcher, in which case we return from the read() now. */ if (lg->dma_is_pending) { if (put_user(lg->pending_dma, user) || put_user(lg->pending_key, user+1)) @@ -335,6 +459,7 @@ int run_guest(struct lguest *lg, unsigned long __user *user) return sizeof(unsigned long)*2; } + /* Check for signals */ if (signal_pending(current)) return -ERESTARTSYS; @@ -342,77 +467,154 @@ int run_guest(struct lguest *lg, unsigned long __user *user) if (lg->break_out) return -EAGAIN; + /* Check if there are any interrupts which can be delivered + * now: if so, this sets up the hander to be executed when we + * next run the Guest. */ maybe_do_interrupt(lg); + /* All long-lived kernel loops need to check with this horrible + * thing called the freezer. If the Host is trying to suspend, + * it stops us. */ try_to_freeze(); + /* Just make absolutely sure the Guest is still alive. One of + * those hypercalls could have been fatal, for example. */ if (lg->dead) break; + /* If the Guest asked to be stopped, we sleep. The Guest's + * clock timer or LHCALL_BREAK from the Waker will wake us. */ if (lg->halted) { set_current_state(TASK_INTERRUPTIBLE); schedule(); continue; } + /* OK, now we're ready to jump into the Guest. First we put up + * the "Do Not Disturb" sign: */ local_irq_disable(); - /* Even if *we* don't want FPU trap, guest might... */ + /* Remember the awfully-named TS bit? If the Guest has asked + * to set it we set it now, so we can trap and pass that trap + * to the Guest if it uses the FPU. */ if (lg->ts) set_ts(); - /* Don't let Guest do SYSENTER: we can't handle it. */ + /* SYSENTER is an optimized way of doing system calls. We + * can't allow it because it always jumps to privilege level 0. + * A normal Guest won't try it because we don't advertise it in + * CPUID, but a malicious Guest (or malicious Guest userspace + * program) could, so we tell the CPU to disable it before + * running the Guest. */ if (boot_cpu_has(X86_FEATURE_SEP)) wrmsr(MSR_IA32_SYSENTER_CS, 0, 0); + /* Now we actually run the Guest. It will pop back out when + * something interesting happens, and we can examine its + * registers to see what it was doing. */ run_guest_once(lg, lguest_pages(raw_smp_processor_id())); - /* Save cr2 now if we page-faulted. */ + /* The "regs" pointer contains two extra entries which are not + * really registers: a trap number which says what interrupt or + * trap made the switcher code come back, and an error code + * which some traps set. */ + + /* If the Guest page faulted, then the cr2 register will tell + * us the bad virtual address. We have to grab this now, + * because once we re-enable interrupts an interrupt could + * fault and thus overwrite cr2, or we could even move off to a + * different CPU. */ if (lg->regs->trapnum == 14) cr2 = read_cr2(); + /* Similarly, if we took a trap because the Guest used the FPU, + * we have to restore the FPU it expects to see. */ else if (lg->regs->trapnum == 7) math_state_restore(); + /* Restore SYSENTER if it's supposed to be on. */ if (boot_cpu_has(X86_FEATURE_SEP)) wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0); + + /* Now we're ready to be interrupted or moved to other CPUs */ local_irq_enable(); + /* OK, so what happened? */ switch (lg->regs->trapnum) { case 13: /* We've intercepted a GPF. */ + /* Check if this was one of those annoying IN or OUT + * instructions which we need to emulate. If so, we + * just go back into the Guest after we've done it. */ if (lg->regs->errcode == 0) { if (emulate_insn(lg)) continue; } break; case 14: /* We've intercepted a page fault. */ + /* The Guest accessed a virtual address that wasn't + * mapped. This happens a lot: we don't actually set + * up most of the page tables for the Guest at all when + * we start: as it runs it asks for more and more, and + * we set them up as required. In this case, we don't + * even tell the Guest that the fault happened. + * + * The errcode tells whether this was a read or a + * write, and whether kernel or userspace code. */ if (demand_page(lg, cr2, lg->regs->errcode)) continue; - /* If lguest_data is NULL, this won't hurt. */ + /* OK, it's really not there (or not OK): the Guest + * needs to know. We write out the cr2 value so it + * knows where the fault occurred. + * + * Note that if the Guest were really messed up, this + * could happen before it's done the INITIALIZE + * hypercall, so lg->lguest_data will be NULL, so + * &lg->lguest_data->cr2 will be address 8. Writing + * into that address won't hurt the Host at all, + * though. */ if (put_user(cr2, &lg->lguest_data->cr2)) kill_guest(lg, "Writing cr2"); break; case 7: /* We've intercepted a Device Not Available fault. */ - /* If they don't want to know, just absorb it. */ + /* If the Guest doesn't want to know, we already + * restored the Floating Point Unit, so we just + * continue without telling it. */ if (!lg->ts) continue; break; - case 32 ... 255: /* Real interrupt, fall thru */ + case 32 ... 255: + /* These values mean a real interrupt occurred, in + * which case the Host handler has already been run. + * We just do a friendly check if another process + * should now be run, then fall through to loop + * around: */ cond_resched(); case LGUEST_TRAP_ENTRY: /* Handled at top of loop */ continue; } + /* If we get here, it's a trap the Guest wants to know + * about. */ if (deliver_trap(lg, lg->regs->trapnum)) continue; + /* If the Guest doesn't have a handler (either it hasn't + * registered any yet, or it's one of the faults we don't let + * it handle), it dies with a cryptic error message. */ kill_guest(lg, "unhandled trap %li at %#lx (%#lx)", lg->regs->trapnum, lg->regs->eip, lg->regs->trapnum == 14 ? cr2 : lg->regs->errcode); } + /* The Guest is dead => "No such file or directory" */ return -ENOENT; } +/* Now we can look at each of the routines this calls, in increasing order of + * complexity: do_hypercalls(), emulate_insn(), maybe_do_interrupt(), + * deliver_trap() and demand_page(). After all those, we'll be ready to + * examine the Switcher, and our philosophical understanding of the Host/Guest + * duality will be complete. :*/ + int find_free_guest(void) { unsigned int i; @@ -430,55 +632,96 @@ static void adjust_pge(void *on) write_cr4(read_cr4() & ~X86_CR4_PGE); } +/*H:000 + * Welcome to the Host! + * + * By this point your brain has been tickled by the Guest code and numbed by + * the Launcher code; prepare for it to be stretched by the Host code. This is + * the heart. Let's begin at the initialization routine for the Host's lg + * module. + */ static int __init init(void) { int err; + /* Lguest can't run under Xen, VMI or itself. It does Tricky Stuff. */ if (paravirt_enabled()) { printk("lguest is afraid of %s\n", paravirt_ops.name); return -EPERM; } + /* First we put the Switcher up in very high virtual memory. */ err = map_switcher(); if (err) return err; + /* Now we set up the pagetable implementation for the Guests. */ err = init_pagetables(switcher_page, SHARED_SWITCHER_PAGES); if (err) { unmap_switcher(); return err; } + + /* The I/O subsystem needs some things initialized. */ lguest_io_init(); + /* /dev/lguest needs to be registered. */ err = lguest_device_init(); if (err) { free_pagetables(); unmap_switcher(); return err; } + + /* Finally, we need to turn off "Page Global Enable". PGE is an + * optimization where page table entries are specially marked to show + * they never change. The Host kernel marks all the kernel pages this + * way because it's always present, even when userspace is running. + * + * Lguest breaks this: unbeknownst to the rest of the Host kernel, we + * switch to the Guest kernel. If you don't disable this on all CPUs, + * you'll get really weird bugs that you'll chase for two days. + * + * I used to turn PGE off every time we switched to the Guest and back + * on when we return, but that slowed the Switcher down noticibly. */ + + /* We don't need the complexity of CPUs coming and going while we're + * doing this. */ lock_cpu_hotplug(); if (cpu_has_pge) { /* We have a broader idea of "global". */ + /* Remember that this was originally set (for cleanup). */ cpu_had_pge = 1; + /* adjust_pge is a helper function which sets or unsets the PGE + * bit on its CPU, depending on the argument (0 == unset). */ on_each_cpu(adjust_pge, (void *)0, 0, 1); + /* Turn off the feature in the global feature set. */ clear_bit(X86_FEATURE_PGE, boot_cpu_data.x86_capability); } unlock_cpu_hotplug(); + + /* All good! */ return 0; } +/* Cleaning up is just the same code, backwards. With a little French. */ static void __exit fini(void) { lguest_device_remove(); free_pagetables(); unmap_switcher(); + + /* If we had PGE before we started, turn it back on now. */ lock_cpu_hotplug(); if (cpu_had_pge) { set_bit(X86_FEATURE_PGE, boot_cpu_data.x86_capability); + /* adjust_pge's argument "1" means set PGE. */ on_each_cpu(adjust_pge, (void *)1, 0, 1); } unlock_cpu_hotplug(); } +/* The Host side of lguest can be a module. This is a nice way for people to + * play with it. */ module_init(init); module_exit(fini); MODULE_LICENSE("GPL"); diff --git a/drivers/lguest/hypercalls.c b/drivers/lguest/hypercalls.c index fb546b0..7a5299f 100644 --- a/drivers/lguest/hypercalls.c +++ b/drivers/lguest/hypercalls.c @@ -28,37 +28,63 @@ #include <irq_vectors.h> #include "lg.h" +/*H:120 This is the core hypercall routine: where the Guest gets what it + * wants. Or gets killed. Or, in the case of LHCALL_CRASH, both. + * + * Remember from the Guest: %eax == which call to make, and the arguments are + * packed into %edx, %ebx and %ecx if needed. */ static void do_hcall(struct lguest *lg, struct lguest_regs *regs) { switch (regs->eax) { case LHCALL_FLUSH_ASYNC: + /* This call does nothing, except by breaking out of the Guest + * it makes us process all the asynchronous hypercalls. */ break; case LHCALL_LGUEST_INIT: + /* You can't get here unless you're already initialized. Don't + * do that. */ kill_guest(lg, "already have lguest_data"); break; case LHCALL_CRASH: { + /* Crash is such a trivial hypercall that we do it in four + * lines right here. */ char msg[128]; + /* If the lgread fails, it will call kill_guest() itself; the + * kill_guest() with the message will be ignored. */ lgread(lg, msg, regs->edx, sizeof(msg)); msg[sizeof(msg)-1] = '\0'; kill_guest(lg, "CRASH: %s", msg); break; } case LHCALL_FLUSH_TLB: + /* FLUSH_TLB comes in two flavors, depending on the + * argument: */ if (regs->edx) guest_pagetable_clear_all(lg); else guest_pagetable_flush_user(lg); break; case LHCALL_GET_WALLCLOCK: { + /* The Guest wants to know the real time in seconds since 1970, + * in good Unix tradition. */ struct timespec ts; ktime_get_real_ts(&ts); regs->eax = ts.tv_sec; break; } case LHCALL_BIND_DMA: + /* BIND_DMA really wants four arguments, but it's the only call + * which does. So the Guest packs the number of buffers and + * the interrupt number into the final argument, and we decode + * it here. This can legitimately fail, since we currently + * place a limit on the number of DMA pools a Guest can have. + * So we return true or false from this call. */ regs->eax = bind_dma(lg, regs->edx, regs->ebx, regs->ecx >> 8, regs->ecx & 0xFF); break; + + /* All these calls simply pass the arguments through to the right + * routines. */ case LHCALL_SEND_DMA: send_dma(lg, regs->edx, regs->ebx); break; @@ -86,10 +112,13 @@ static void do_hcall(struct lguest *lg, struct lguest_regs *regs) case LHCALL_SET_CLOCKEVENT: guest_set_clockevent(lg, regs->edx); break; + case LHCALL_TS: + /* This sets the TS flag, as we saw used in run_guest(). */ lg->ts = regs->edx; break; case LHCALL_HALT: + /* Similarly, this sets the halted flag for run_guest(). */ lg->halted = 1; break; default: @@ -97,25 +126,42 @@ static void do_hcall(struct lguest *lg, struct lguest_regs *regs) } } -/* We always do queued calls before actual hypercall. */ +/* Asynchronous hypercalls are easy: we just look in the array in the Guest's + * "struct lguest_data" and see if there are any new ones marked "ready". + * + * We are careful to do these in order: obviously we respect the order the + * Guest put them in the ring, but we also promise the Guest that they will + * happen before any normal hypercall (which is why we check this before + * checking for a normal hcall). */ static void do_async_hcalls(struct lguest *lg) { unsigned int i; u8 st[LHCALL_RING_SIZE]; + /* For simplicity, we copy the entire call status array in at once. */ if (copy_from_user(&st, &lg->lguest_data->hcall_status, sizeof(st))) return; + + /* We process "struct lguest_data"s hcalls[] ring once. */ for (i = 0; i < ARRAY_SIZE(st); i++) { struct lguest_regs regs; + /* We remember where we were up to from last time. This makes + * sure that the hypercalls are done in the order the Guest + * places them in the ring. */ unsigned int n = lg->next_hcall; + /* 0xFF means there's no call here (yet). */ if (st[n] == 0xFF) break; + /* OK, we have hypercall. Increment the "next_hcall" cursor, + * and wrap back to 0 if we reach the end. */ if (++lg->next_hcall == LHCALL_RING_SIZE) lg->next_hcall = 0; + /* We copy the hypercall arguments into a fake register + * structure. This makes life simple for do_hcall(). */ if (get_user(regs.eax, &lg->lguest_data->hcalls[n].eax) || get_user(regs.edx, &lg->lguest_data->hcalls[n].edx) || get_user(regs.ecx, &lg->lguest_data->hcalls[n].ecx) @@ -124,74 +170,126 @@ static void do_async_hcalls(struct lguest *lg) break; } + /* Do the hypercall, same as a normal one. */ do_hcall(lg, ®s); + + /* Mark the hypercall done. */ if (put_user(0xFF, &lg->lguest_data->hcall_status[n])) { kill_guest(lg, "Writing result for async hypercall"); break; } + /* Stop doing hypercalls if we've just done a DMA to the + * Launcher: it needs to service this first. */ if (lg->dma_is_pending) break; } } +/* Last of all, we look at what happens first of all. The very first time the + * Guest makes a hypercall, we end up here to set things up: */ static void initialize(struct lguest *lg) { u32 tsc_speed; + /* You can't do anything until you're initialized. The Guest knows the + * rules, so we're unforgiving here. */ if (lg->regs->eax != LHCALL_LGUEST_INIT) { kill_guest(lg, "hypercall %li before LGUEST_INIT", lg->regs->eax); return; } - /* We only tell the guest to use the TSC if it's reliable. */ + /* We insist that the Time Stamp Counter exist and doesn't change with + * cpu frequency. Some devious chip manufacturers decided that TSC + * changes could be handled in software. I decided that time going + * backwards might be good for benchmarks, but it's bad for users. + * + * We also insist that the TSC be stable: the kernel detects unreliable + * TSCs for its own purposes, and we use that here. */ if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) && !check_tsc_unstable()) tsc_speed = tsc_khz; else tsc_speed = 0; + /* The pointer to the Guest's "struct lguest_data" is the only + * argument. */ lg->lguest_data = (struct lguest_data __user *)lg->regs->edx; - /* We check here so we can simply copy_to_user/from_user */ + /* If we check the address they gave is OK now, we can simply + * copy_to_user/from_user from now on rather than using lgread/lgwrite. + * I put this in to show that I'm not immune to writing stupid + * optimizations. */ if (!lguest_address_ok(lg, lg->regs->edx, sizeof(*lg->lguest_data))) { kill_guest(lg, "bad guest page %p", lg->lguest_data); return; } + /* The Guest tells us where we're not to deliver interrupts by putting + * the range of addresses into "struct lguest_data". */ if (get_user(lg->noirq_start, &lg->lguest_data->noirq_start) || get_user(lg->noirq_end, &lg->lguest_data->noirq_end) - /* We reserve the top pgd entry. */ + /* We tell the Guest that it can't use the top 4MB of virtual + * addresses used by the Switcher. */ || put_user(4U*1024*1024, &lg->lguest_data->reserve_mem) || put_user(tsc_speed, &lg->lguest_data->tsc_khz) + /* We also give the Guest a unique id, as used in lguest_net.c. */ || put_user(lg->guestid, &lg->lguest_data->guestid)) kill_guest(lg, "bad guest page %p", lg->lguest_data); - /* This is the one case where the above accesses might have - * been the first write to a Guest page. This may have caused - * a copy-on-write fault, but the Guest might be referring to - * the old (read-only) page. */ + /* This is the one case where the above accesses might have been the + * first write to a Guest page. This may have caused a copy-on-write + * fault, but the Guest might be referring to the old (read-only) + * page. */ guest_pagetable_clear_all(lg); } +/* Now we've examined the hypercall code; our Guest can make requests. There + * is one other way we can do things for the Guest, as we see in + * emulate_insn(). */ -/* Even if we go out to userspace and come back, we don't want to do - * the hypercall again. */ +/*H:110 Tricky point: we mark the hypercall as "done" once we've done it. + * Normally we don't need to do this: the Guest will run again and update the + * trap number before we come back around the run_guest() loop to + * do_hypercalls(). + * + * However, if we are signalled or the Guest sends DMA to the Launcher, that + * loop will exit without running the Guest. When it comes back it would try + * to re-run the hypercall. */ static void clear_hcall(struct lguest *lg) { lg->regs->trapnum = 255; } +/*H:100 + * Hypercalls + * + * Remember from the Guest, hypercalls come in two flavors: normal and + * asynchronous. This file handles both of types. + */ void do_hypercalls(struct lguest *lg) { + /* Not initialized yet? */ if (unlikely(!lg->lguest_data)) { + /* Did the Guest make a hypercall? We might have come back for + * some other reason (an interrupt, a different trap). */ if (lg->regs->trapnum == LGUEST_TRAP_ENTRY) { + /* Set up the "struct lguest_data" */ initialize(lg); + /* The hypercall is done. */ clear_hcall(lg); } return; } + /* The Guest has initialized. + * + * Look in the hypercall ring for the async hypercalls: */ do_async_hcalls(lg); + + /* If we stopped reading the hypercall ring because the Guest did a + * SEND_DMA to the Launcher, we want to return now. Otherwise if the + * Guest asked us to do a hypercall, we do it. */ if (!lg->dma_is_pending && lg->regs->trapnum == LGUEST_TRAP_ENTRY) { do_hcall(lg, lg->regs); + /* The hypercall is done. */ clear_hcall(lg); } } diff --git a/drivers/lguest/interrupts_and_traps.c b/drivers/lguest/interrupts_and_traps.c index b264797..3d98303 100644 --- a/drivers/lguest/interrupts_and_traps.c +++ b/drivers/lguest/interrupts_and_traps.c @@ -14,100 +14,147 @@ #include <linux/uaccess.h> #include "lg.h" +/* The address of the interrupt handler is split into two bits: */ static unsigned long idt_address(u32 lo, u32 hi) { return (lo & 0x0000FFFF) | (hi & 0xFFFF0000); } +/* The "type" of the interrupt handler is a 4 bit field: we only support a + * couple of types. */ static int idt_type(u32 lo, u32 hi) { return (hi >> 8) & 0xF; } +/* An IDT entry can't be used unless the "present" bit is set. */ static int idt_present(u32 lo, u32 hi) { return (hi & 0x8000); } +/* We need a helper to "push" a value onto the Guest's stack, since that's a + * big part of what delivering an interrupt does. */ static void push_guest_stack(struct lguest *lg, unsigned long *gstack, u32 val) { + /* Stack grows upwards: move stack then write value. */ *gstack -= 4; lgwrite_u32(lg, *gstack, val); } +/*H:210 The set_guest_interrupt() routine actually delivers the interrupt or + * trap. The mechanics of delivering traps and interrupts to the Guest are the + * same, except some traps have an "error code" which gets pushed onto the + * stack as well: the caller tells us if this is one. + * + * "lo" and "hi" are the two parts of the Interrupt Descriptor Table for this + * interrupt or trap. It's split into two parts for traditional reasons: gcc + * on i386 used to be frightened by 64 bit numbers. + * + * We set up the stack just like the CPU does for a real interrupt, so it's + * identical for the Guest (and the standard "iret" instruction will undo + * it). */ static void set_guest_interrupt(struct lguest *lg, u32 lo, u32 hi, int has_err) { unsigned long gstack; u32 eflags, ss, irq_enable; - /* If they want a ring change, we use new stack and push old ss/esp */ + /* There are two cases for interrupts: one where the Guest is already + * in the kernel, and a more complex one where the Guest is in + * userspace. We check the privilege level to find out. */ if ((lg->regs->ss&0x3) != GUEST_PL) { + /* The Guest told us their kernel stack with the SET_STACK + * hypercall: both the virtual address and the segment */ gstack = guest_pa(lg, lg->esp1); ss = lg->ss1; + /* We push the old stack segment and pointer onto the new + * stack: when the Guest does an "iret" back from the interrupt + * handler the CPU will notice they're dropping privilege + * levels and expect these here. */ push_guest_stack(lg, &gstack, lg->regs->ss); push_guest_stack(lg, &gstack, lg->regs->esp); } else { + /* We're staying on the same Guest (kernel) stack. */ gstack = guest_pa(lg, lg->regs->esp); ss = lg->regs->ss; } - /* We use IF bit in eflags to indicate whether irqs were enabled - (it's always 1, since irqs are enabled when guest is running). */ + /* Remember that we never let the Guest actually disable interrupts, so + * the "Interrupt Flag" bit is always set. We copy that bit from the + * Guest's "irq_enabled" field into the eflags word: the Guest copies + * it back in "lguest_iret". */ eflags = lg->regs->eflags; if (get_user(irq_enable, &lg->lguest_data->irq_enabled) == 0 && !(irq_enable & X86_EFLAGS_IF)) eflags &= ~X86_EFLAGS_IF; + /* An interrupt is expected to push three things on the stack: the old + * "eflags" word, the old code segment, and the old instruction + * pointer. */ push_guest_stack(lg, &gstack, eflags); push_guest_stack(lg, &gstack, lg->regs->cs); push_guest_stack(lg, &gstack, lg->regs->eip); + /* For the six traps which supply an error code, we push that, too. */ if (has_err) push_guest_stack(lg, &gstack, lg->regs->errcode); - /* Change the real stack so switcher returns to trap handler */ + /* Now we've pushed all the old state, we change the stack, the code + * segment and the address to execute. */ lg->regs->ss = ss; lg->regs->esp = gstack + lg->page_offset; lg->regs->cs = (__KERNEL_CS|GUEST_PL); lg->regs->eip = idt_address(lo, hi); - /* Disable interrupts for an interrupt gate. */ + /* There are two kinds of interrupt handlers: 0xE is an "interrupt + * gate" which expects interrupts to be disabled on entry. */ if (idt_type(lo, hi) == 0xE) if (put_user(0, &lg->lguest_data->irq_enabled)) kill_guest(lg, "Disabling interrupts"); } +/*H:200 + * Virtual Interrupts. + * + * maybe_do_interrupt() gets called before every entry to the Guest, to see if + * we should divert the Guest to running an interrupt handler. */ void maybe_do_interrupt(struct lguest *lg) { unsigned int irq; DECLARE_BITMAP(blk, LGUEST_IRQS); struct desc_struct *idt; + /* If the Guest hasn't even initialized yet, we can do nothing. */ if (!lg->lguest_data) return; - /* Mask out any interrupts they have blocked. */ + /* Take our "irqs_pending" array and remove any interrupts the Guest + * wants blocked: the result ends up in "blk". */ if (copy_from_user(&blk, lg->lguest_data->blocked_interrupts, sizeof(blk))) return; bitmap_andnot(blk, lg->irqs_pending, blk, LGUEST_IRQS); + /* Find the first interrupt. */ irq = find_first_bit(blk, LGUEST_IRQS); + /* None? Nothing to do */ if (irq >= LGUEST_IRQS) return; + /* They may be in the middle of an iret, where they asked us never to + * deliver interrupts. */ if (lg->regs->eip >= lg->noirq_start && lg->regs->eip < lg->noirq_end) return; - /* If they're halted, we re-enable interrupts. */ + /* If they're halted, interrupts restart them. */ if (lg->halted) { /* Re-enable interrupts. */ if (put_user(X86_EFLAGS_IF, &lg->lguest_data->irq_enabled)) kill_guest(lg, "Re-enabling interrupts"); lg->halted = 0; } else { - /* Maybe they have interrupts disabled? */ + /* Otherwise we check if they have interrupts disabled. */ u32 irq_enabled; if (get_user(irq_enabled, &lg->lguest_data->irq_enabled)) irq_enabled = 0; @@ -115,112 +162,197 @@ void maybe_do_interrupt(struct lguest *lg) return; } + /* Look at the IDT entry the Guest gave us for this interrupt. The + * first 32 (FIRST_EXTERNAL_VECTOR) entries are for traps, so we skip + * over them. */ idt = &lg->idt[FIRST_EXTERNAL_VECTOR+irq]; + /* If they don't have a handler (yet?), we just ignore it */ if (idt_present(idt->a, idt->b)) { + /* OK, mark it no longer pending and deliver it. */ clear_bit(irq, lg->irqs_pending); + /* set_guest_interrupt() takes the interrupt descriptor and a + * flag to say whether this interrupt pushes an error code onto + * the stack as well: virtual interrupts never do. */ set_guest_interrupt(lg, idt->a, idt->b, 0); } } +/*H:220 Now we've got the routines to deliver interrupts, delivering traps + * like page fault is easy. The only trick is that Intel decided that some + * traps should have error codes: */ static int has_err(unsigned int trap) { return (trap == 8 || (trap >= 10 && trap <= 14) || trap == 17); } +/* deliver_trap() returns true if it could deliver the trap. */ int deliver_trap(struct lguest *lg, unsigned int num) { u32 lo = lg->idt[num].a, hi = lg->idt[num].b; + /* Early on the Guest hasn't set the IDT entries (or maybe it put a + * bogus one in): if we fail here, the Guest will be killed. */ if (!idt_present(lo, hi)) return 0; set_guest_interrupt(lg, lo, hi, has_err(num)); return 1; } +/*H:250 Here's the hard part: returning to the Host every time a trap happens + * and then calling deliver_trap() and re-entering the Guest is slow. + * Particularly because Guest userspace system calls are traps (trap 128). + * + * So we'd like to set up the IDT to tell the CPU to deliver traps directly + * into the Guest. This is possible, but the complexities cause the size of + * this file to double! However, 150 lines of code is worth writing for taking + * system calls down from 1750ns to 270ns. Plus, if lguest didn't do it, all + * the other hypervisors would tease it. + * + * This routine determines if a trap can be delivered directly. */ static int direct_trap(const struct lguest *lg, const struct desc_struct *trap, unsigned int num) { - /* Hardware interrupts don't go to guest (except syscall). */ + /* Hardware interrupts don't go to the Guest at all (except system + * call). */ if (num >= FIRST_EXTERNAL_VECTOR && num != SYSCALL_VECTOR) return 0; - /* We intercept page fault (demand shadow paging & cr2 saving) - protection fault (in/out emulation) and device not - available (TS handling), and hypercall */ + /* The Host needs to see page faults (for shadow paging and to save the + * fault address), general protection faults (in/out emulation) and + * device not available (TS handling), and of course, the hypercall + * trap. */ if (num == 14 || num == 13 || num == 7 || num == LGUEST_TRAP_ENTRY) return 0; - /* Interrupt gates (0xE) or not present (0x0) can't go direct. */ + /* Only trap gates (type 15) can go direct to the Guest. Interrupt + * gates (type 14) disable interrupts as they are entered, which we + * never let the Guest do. Not present entries (type 0x0) also can't + * go direct, of course 8) */ return idt_type(trap->a, trap->b) == 0xF; } +/*H:260 When we make traps go directly into the Guest, we need to make sure + * the kernel stack is valid (ie. mapped in the page tables). Otherwise, the + * CPU trying to deliver the trap will fault while trying to push the interrupt + * words on the stack: this is called a double fault, and it forces us to kill + * the Guest. + * + * Which is deeply unfair, because (literally!) it wasn't the Guests' fault. */ void pin_stack_pages(struct lguest *lg) { unsigned int i; + /* Depending on the CONFIG_4KSTACKS option, the Guest can have one or + * two pages of stack space. */ for (i = 0; i < lg->stack_pages; i++) + /* The stack grows *upwards*, hence the subtraction */ pin_page(lg, lg->esp1 - i * PAGE_SIZE); } +/* Direct traps also mean that we need to know whenever the Guest wants to use + * a different kernel stack, so we can change the IDT entries to use that + * stack. The IDT entries expect a virtual address, so unlike most addresses + * the Guest gives us, the "esp" (stack pointer) value here is virtual, not + * physical. + * + * In Linux each process has its own kernel stack, so this happens a lot: we + * change stacks on each context switch. */ void guest_set_stack(struct lguest *lg, u32 seg, u32 esp, unsigned int pages) { - /* You cannot have a stack segment with priv level 0. */ + /* You are not allowd have a stack segment with privilege level 0: bad + * Guest! */ if ((seg & 0x3) != GUEST_PL) kill_guest(lg, "bad stack segment %i", seg); + /* We only expect one or two stack pages. */ if (pages > 2) kill_guest(lg, "bad stack pages %u", pages); + /* Save where the stack is, and how many pages */ lg->ss1 = seg; lg->esp1 = esp; lg->stack_pages = pages; + /* Make sure the new stack pages are mapped */ pin_stack_pages(lg); } -/* Set up trap in IDT. */ +/* All this reference to mapping stacks leads us neatly into the other complex + * part of the Host: page table handling. */ + +/*H:235 This is the routine which actually checks the Guest's IDT entry and + * transfers it into our entry in "struct lguest": */ static void set_trap(struct lguest *lg, struct desc_struct *trap, unsigned int num, u32 lo, u32 hi) { u8 type = idt_type(lo, hi); + /* We zero-out a not-present entry */ if (!idt_present(lo, hi)) { trap->a = trap->b = 0; return; } + /* We only support interrupt and trap gates. */ if (type != 0xE && type != 0xF) kill_guest(lg, "bad IDT type %i", type); + /* We only copy the handler address, present bit, privilege level and + * type. The privilege level controls where the trap can be triggered + * manually with an "int" instruction. This is usually GUEST_PL, + * except for system calls which userspace can use. */ trap->a = ((__KERNEL_CS|GUEST_PL)<<16) | (lo&0x0000FFFF); trap->b = (hi&0xFFFFEF00); } +/*H:230 While we're here, dealing with delivering traps and interrupts to the + * Guest, we might as well complete the picture: how the Guest tells us where + * it wants them to go. This would be simple, except making traps fast + * requires some tricks. + * + * We saw the Guest setting Interrupt Descriptor Table (IDT) entries with the + * LHCALL_LOAD_IDT_ENTRY hypercall before: that comes here. */ void load_guest_idt_entry(struct lguest *lg, unsigned int num, u32 lo, u32 hi) { - /* Guest never handles: NMI, doublefault, hypercall, spurious irq. */ + /* Guest never handles: NMI, doublefault, spurious interrupt or + * hypercall. We ignore when it tries to set them. */ if (num == 2 || num == 8 || num == 15 || num == LGUEST_TRAP_ENTRY) return; + /* Mark the IDT as changed: next time the Guest runs we'll know we have + * to copy this again. */ lg->changed |= CHANGED_IDT; + + /* The IDT which we keep in "struct lguest" only contains 32 entries + * for the traps and LGUEST_IRQS (32) entries for interrupts. We + * ignore attempts to set handlers for higher interrupt numbers, except + * for the system call "interrupt" at 128: we have a special IDT entry + * for that. */ if (num < ARRAY_SIZE(lg->idt)) set_trap(lg, &lg->idt[num], num, lo, hi); else if (num == SYSCALL_VECTOR) set_trap(lg, &lg->syscall_idt, num, lo, hi); } +/* The default entry for each interrupt points into the Switcher routines which + * simply return to the Host. The run_guest() loop will then call + * deliver_trap() to bounce it back into the Guest. */ static void default_idt_entry(struct desc_struct *idt, int trap, const unsigned long handler) { + /* A present interrupt gate. */ u32 flags = 0x8e00; - /* They can't "int" into any of them except hypercall. */ + /* Set the privilege level on the entry for the hypercall: this allows + * the Guest to use the "int" instruction to trigger it. */ if (trap == LGUEST_TRAP_ENTRY) flags |= (GUEST_PL << 13); + /* Now pack it into the IDT entry in its weird format. */ idt->a = (LGUEST_CS<<16) | (handler&0x0000FFFF); idt->b = (handler&0xFFFF0000) | flags; } +/* When the Guest first starts, we put default entries into the IDT. */ void setup_default_idt_entries(struct lguest_ro_state *state, const unsigned long *def) { @@ -230,19 +362,25 @@ void setup_default_idt_entries(struct lguest_ro_state *state, default_idt_entry(&state->guest_idt[i], i, def[i]); } +/*H:240 We don't use the IDT entries in the "struct lguest" directly, instead + * we copy them into the IDT which we've set up for Guests on this CPU, just + * before we run the Guest. This routine does that copy. */ void copy_traps(const struct lguest *lg, struct desc_struct *idt, const unsigned long *def) { unsigned int i; - /* All hardware interrupts are same whatever the guest: only the - * traps might be different. */ + /* We can simply copy the direct traps, otherwise we use the default + * ones in the Switcher: they will return to the Host. */ for (i = 0; i < FIRST_EXTERNAL_VECTOR; i++) { if (direct_trap(lg, &lg->idt[i], i)) idt[i] = lg->idt[i]; else default_idt_entry(&idt[i], i, def[i]); } + + /* Don't forget the system call trap! The IDT entries for other + * interupts never change, so no need to copy them. */ i = SYSCALL_VECTOR; if (direct_trap(lg, &lg->syscall_idt, i)) idt[i] = lg->syscall_idt; diff --git a/drivers/lguest/lg.h b/drivers/lguest/lg.h index 3b9dc12..269116e 100644 --- a/drivers/lguest/lg.h +++ b/drivers/lguest/lg.h @@ -58,9 +58,18 @@ struct lguest_dma_info u8 interrupt; /* 0 when not registered */ }; -/* We have separate types for the guest's ptes & pgds and the shadow ptes & - * pgds. Since this host might use three-level pagetables and the guest and - * shadow pagetables don't, we can't use the normal pte_t/pgd_t. */ +/*H:310 The page-table code owes a great debt of gratitude to Andi Kleen. He + * reviewed the original code which used "u32" for all page table entries, and + * insisted that it would be far clearer with explicit typing. I thought it + * was overkill, but he was right: it is much clearer than it was before. + * + * We have separate types for the Guest's ptes & pgds and the shadow ptes & + * pgds. There's already a Linux type for these (pte_t and pgd_t) but they + * change depending on kernel config options (PAE). */ + +/* Each entry is identical: lower 12 bits of flags and upper 20 bits for the + * "page frame number" (0 == first physical page, etc). They are different + * types so the compiler will warn us if we mix them improperly. */ typedef union { struct { unsigned flags:12, pfn:20; }; struct { unsigned long val; } raw; @@ -77,8 +86,12 @@ typedef union { struct { unsigned flags:12, pfn:20; }; struct { unsigned long val; } raw; } gpte_t; + +/* We have two convenient macros to convert a "raw" value as handed to us by + * the Guest into the correct Guest PGD or PTE type. */ #define mkgpte(_val) ((gpte_t){.raw.val = _val}) #define mkgpgd(_val) ((gpgd_t){.raw.val = _val}) +/*:*/ struct pgdir { diff --git a/drivers/lguest/page_tables.c b/drivers/lguest/page_tables.c index f9ca50d..cd047e8 100644 --- a/drivers/lguest/page_tables.c +++ b/drivers/lguest/page_tables.c @@ -15,38 +15,91 @@ #include <asm/tlbflush.h> #include "lg.h" +/*H:300 + * The Page Table Code + * + * We use two-level page tables for the Guest. If you're not entirely + * comfortable with virtual addresses, physical addresses and page tables then + * I recommend you review lguest.c's "Page Table Handling" (with diagrams!). + * + * The Guest keeps page tables, but we maintain the actual ones here: these are + * called "shadow" page tables. Which is a very Guest-centric name: these are + * the real page tables the CPU uses, although we keep them up to date to + * reflect the Guest's. (See what I mean about weird naming? Since when do + * shadows reflect anything?) + * + * Anyway, this is the most complicated part of the Host code. There are seven + * parts to this: + * (i) Setting up a page table entry for the Guest when it faults, + * (ii) Setting up the page table entry for the Guest stack, + * (iii) Setting up a page table entry when the Guest tells us it has changed, + * (iv) Switching page tables, + * (v) Flushing (thowing away) page tables, + * (vi) Mapping the Switcher when the Guest is about to run, + * (vii) Setting up the page tables initially. + :*/ + +/* Pages a 4k long, and each page table entry is 4 bytes long, giving us 1024 + * (or 2^10) entries per page. */ #define PTES_PER_PAGE_SHIFT 10 #define PTES_PER_PAGE (1 << PTES_PER_PAGE_SHIFT) + +/* 1024 entries in a page table page maps 1024 pages: 4MB. The Switcher is + * conveniently placed at the top 4MB, so it uses a separate, complete PTE + * page. */ #define SWITCHER_PGD_INDEX (PTES_PER_PAGE - 1) +/* We actually need a separate PTE page for each CPU. Remember that after the + * Switcher code itself comes two pages for each CPU, and we don't want this + * CPU's guest to see the pages of any other CPU. */ static DEFINE_PER_CPU(spte_t *, switcher_pte_pages); #define switcher_pte_page(cpu) per_cpu(switcher_pte_pages, cpu) +/*H:320 With our shadow and Guest types established, we need to deal with + * them: the page table code is curly enough to need helper functions to keep + * it clear and clean. + * + * The first helper takes a virtual address, and says which entry in the top + * level page table deals with that address. Since each top level entry deals + * with 4M, this effectively divides by 4M. */ static unsigned vaddr_to_pgd_index(unsigned long vaddr) { return vaddr >> (PAGE_SHIFT + PTES_PER_PAGE_SHIFT); } -/* These access the shadow versions (ie. the ones used by the CPU). */ +/* There are two functions which return pointers to the shadow (aka "real") + * page tables. + * + * spgd_addr() takes the virtual address and returns a pointer to the top-level + * page directory entry for that address. Since we keep track of several page + * tables, the "i" argument tells us which one we're interested in (it's + * usually the current one). */ static spgd_t *spgd_addr(struct lguest *lg, u32 i, unsigned long vaddr) { unsigned int index = vaddr_to_pgd_index(vaddr); + /* We kill any Guest trying to touch the Switcher addresses. */ if (index >= SWITCHER_PGD_INDEX) { kill_guest(lg, "attempt to access switcher pages"); index = 0; } + /* Return a pointer index'th pgd entry for the i'th page table. */ return &lg->pgdirs[i].pgdir[index]; } +/* This routine then takes the PGD entry given above, which contains the + * address of the PTE page. It then returns a pointer to the PTE entry for the + * given address. */ static spte_t *spte_addr(struct lguest *lg, spgd_t spgd, unsigned long vaddr) { spte_t *page = __va(spgd.pfn << PAGE_SHIFT); + /* You should never call this if the PGD entry wasn't valid */ BUG_ON(!(spgd.flags & _PAGE_PRESENT)); return &page[(vaddr >> PAGE_SHIFT) % PTES_PER_PAGE]; } -/* These access the guest versions. */ +/* These two functions just like the above two, except they access the Guest + * page tables. Hence they return a Guest address. */ static unsigned long gpgd_addr(struct lguest *lg, unsigned long vaddr) { unsigned int index = vaddr >> (PAGE_SHIFT + PTES_PER_PAGE_SHIFT); @@ -61,12 +114,24 @@ static unsigned long gpte_addr(struct lguest *lg, return gpage + ((vaddr>>PAGE_SHIFT) % PTES_PER_PAGE) * sizeof(gpte_t); } -/* Do a virtual -> physical mapping on a user page. */ +/*H:350 This routine takes a page number given by the Guest and converts it to + * an actual, physical page number. It can fail for several reasons: the + * virtual address might not be mapped by the Launcher, the write flag is set + * and the page is read-only, or the write flag was set and the page was + * shared so had to be copied, but we ran out of memory. + * + * This holds a reference to the page, so release_pte() is careful to + * put that back. */ static unsigned long get_pfn(unsigned long virtpfn, int write) { struct page *page; + /* This value indicates failure. */ unsigned long ret = -1UL; + /* get_user_pages() is a complex interface: it gets the "struct + * vm_area_struct" and "struct page" assocated with a range of pages. + * It also needs the task's mmap_sem held, and is not very quick. + * It returns the number of pages it got. */ down_read(¤t->mm->mmap_sem); if (get_user_pages(current, current->mm, virtpfn << PAGE_SHIFT, 1, write, 1, &page, NULL) == 1) @@ -75,28 +140,47 @@ static unsigned long get_pfn(unsigned long virtpfn, int write) return ret; } +/*H:340 Converting a Guest page table entry to a shadow (ie. real) page table + * entry can be a little tricky. The flags are (almost) the same, but the + * Guest PTE contains a virtual page number: the CPU needs the real page + * number. */ static spte_t gpte_to_spte(struct lguest *lg, gpte_t gpte, int write) { spte_t spte; unsigned long pfn; - /* We ignore the global flag. */ + /* The Guest sets the global flag, because it thinks that it is using + * PGE. We only told it to use PGE so it would tell us whether it was + * flushing a kernel mapping or a userspace mapping. We don't actually + * use the global bit, so throw it away. */ spte.flags = (gpte.flags & ~_PAGE_GLOBAL); + + /* We need a temporary "unsigned long" variable to hold the answer from + * get_pfn(), because it returns 0xFFFFFFFF on failure, which wouldn't + * fit in spte.pfn. get_pfn() finds the real physical number of the + * page, given the virtual number. */ pfn = get_pfn(gpte.pfn, write); if (pfn == -1UL) { kill_guest(lg, "failed to get page %u", gpte.pfn); - /* Must not put_page() bogus page on cleanup. */ + /* When we destroy the Guest, we'll go through the shadow page + * tables and release_pte() them. Make sure we don't think + * this one is valid! */ spte.flags = 0; } + /* Now we assign the page number, and our shadow PTE is complete. */ spte.pfn = pfn; return spte; } +/*H:460 And to complete the chain, release_pte() looks like this: */ static void release_pte(spte_t pte) { + /* Remember that get_user_pages() took a reference to the page, in + * get_pfn()? We have to put it back now. */ if (pte.flags & _PAGE_PRESENT) put_page(pfn_to_page(pte.pfn)); } +/*:*/ static void check_gpte(struct lguest *lg, gpte_t gpte) { @@ -110,11 +194,16 @@ static void check_gpgd(struct lguest *lg, gpgd_t gpgd) kill_guest(lg, "bad page directory entry"); } -/* FIXME: We hold reference to pages, which prevents them from being - swapped. It'd be nice to have a callback when Linux wants to swap out. */ - -/* We fault pages in, which allows us to update accessed/dirty bits. - * Return true if we got page. */ +/*H:330 + * (i) Setting up a page table entry for the Guest when it faults + * + * We saw this call in run_guest(): when we see a page fault in the Guest, we + * come here. That's because we only set up the shadow page tables lazily as + * they're needed, so we get page faults all the time and quietly fix them up + * and return to the Guest without it knowing. + * + * If we fixed up the fault (ie. we mapped the address), this routine returns + * true. */ int demand_page(struct lguest *lg, unsigned long vaddr, int errcode) { gpgd_t gpgd; @@ -123,106 +212,161 @@ int demand_page(struct lguest *lg, unsigned long vaddr, int errcode) gpte_t gpte; spte_t *spte; + /* First step: get the top-level Guest page table entry. */ gpgd = mkgpgd(lgread_u32(lg, gpgd_addr(lg, vaddr))); + /* Toplevel not present? We can't map it in. */ if (!(gpgd.flags & _PAGE_PRESENT)) return 0; + /* Now look at the matching shadow entry. */ spgd = spgd_addr(lg, lg->pgdidx, vaddr); if (!(spgd->flags & _PAGE_PRESENT)) { - /* Get a page of PTEs for them. */ + /* No shadow entry: allocate a new shadow PTE page. */ unsigned long ptepage = get_zeroed_page(GFP_KERNEL); - /* FIXME: Steal from self in this case? */ + /* This is not really the Guest's fault, but killing it is + * simple for this corner case. */ if (!ptepage) { kill_guest(lg, "out of memory allocating pte page"); return 0; } + /* We check that the Guest pgd is OK. */ check_gpgd(lg, gpgd); + /* And we copy the flags to the shadow PGD entry. The page + * number in the shadow PGD is the page we just allocated. */ spgd->raw.val = (__pa(ptepage) | gpgd.flags); } + /* OK, now we look at the lower level in the Guest page table: keep its + * address, because we might update it later. */ gpte_ptr = gpte_addr(lg, gpgd, vaddr); gpte = mkgpte(lgread_u32(lg, gpte_ptr)); - /* No page? */ + /* If this page isn't in the Guest page tables, we can't page it in. */ if (!(gpte.flags & _PAGE_PRESENT)) return 0; - /* Write to read-only page? */ + /* Check they're not trying to write to a page the Guest wants + * read-only (bit 2 of errcode == write). */ if ((errcode & 2) && !(gpte.flags & _PAGE_RW)) return 0; - /* User access to a non-user page? */ + /* User access to a kernel page? (bit 3 == user access) */ if ((errcode & 4) && !(gpte.flags & _PAGE_USER)) return 0; + /* Check that the Guest PTE flags are OK, and the page number is below + * the pfn_limit (ie. not mapping the Launcher binary). */ check_gpte(lg, gpte); + /* Add the _PAGE_ACCESSED and (for a write) _PAGE_DIRTY flag */ gpte.flags |= _PAGE_ACCESSED; if (errcode & 2) gpte.flags |= _PAGE_DIRTY; - /* We're done with the old pte. */ + /* Get the pointer to the shadow PTE entry we're going to set. */ spte = spte_addr(lg, *spgd, vaddr); + /* If there was a valid shadow PTE entry here before, we release it. + * This can happen with a write to a previously read-only entry. */ release_pte(*spte); - /* We don't make it writable if this isn't a write: later - * write will fault so we can set dirty bit in guest. */ + /* If this is a write, we insist that the Guest page is writable (the + * final arg to gpte_to_spte()). */ if (gpte.flags & _PAGE_DIRTY) *spte = gpte_to_spte(lg, gpte, 1); else { + /* If this is a read, don't set the "writable" bit in the page + * table entry, even if the Guest says it's writable. That way + * we come back here when a write does actually ocur, so we can + * update the Guest's _PAGE_DIRTY flag. */ gpte_t ro_gpte = gpte; ro_gpte.flags &= ~_PAGE_RW; *spte = gpte_to_spte(lg, ro_gpte, 0); } - /* Now we update dirty/accessed on guest. */ + /* Finally, we write the Guest PTE entry back: we've set the + * _PAGE_ACCESSED and maybe the _PAGE_DIRTY flags. */ lgwrite_u32(lg, gpte_ptr, gpte.raw.val); + + /* We succeeded in mapping the page! */ return 1; } -/* This is much faster than the full demand_page logic. */ +/*H:360 (ii) Setting up the page table entry for the Guest stack. + * + * Remember pin_stack_pages() which makes sure the stack is mapped? It could + * simply call demand_page(), but as we've seen that logic is quite long, and + * usually the stack pages are already mapped anyway, so it's not required. + * + * This is a quick version which answers the question: is this virtual address + * mapped by the shadow page tables, and is it writable? */ static int page_writable(struct lguest *lg, unsigned long vaddr) { spgd_t *spgd; unsigned long flags; + /* Look at the top level entry: is it present? */ spgd = spgd_addr(lg, lg->pgdidx, vaddr); if (!(spgd->flags & _PAGE_PRESENT)) return 0; + /* Check the flags on the pte entry itself: it must be present and + * writable. */ flags = spte_addr(lg, *spgd, vaddr)->flags; return (flags & (_PAGE_PRESENT|_PAGE_RW)) == (_PAGE_PRESENT|_PAGE_RW); } +/* So, when pin_stack_pages() asks us to pin a page, we check if it's already + * in the page tables, and if not, we call demand_page() with error code 2 + * (meaning "write"). */ void pin_page(struct lguest *lg, unsigned long vaddr) { if (!page_writable(lg, vaddr) && !demand_page(lg, vaddr, 2)) kill_guest(lg, "bad stack page %#lx", vaddr); } +/*H:450 If we chase down the release_pgd() code, it looks like this: */ static void release_pgd(struct lguest *lg, spgd_t *spgd) { + /* If the entry's not present, there's nothing to release. */ if (spgd->flags & _PAGE_PRESENT) { unsigned int i; + /* Converting the pfn to find the actual PTE page is easy: turn + * the page number into a physical address, then convert to a + * virtual address (easy for kernel pages like this one). */ spte_t *ptepage = __va(spgd->pfn << PAGE_SHIFT); + /* For each entry in the page, we might need to release it. */ for (i = 0; i < PTES_PER_PAGE; i++) release_pte(ptepage[i]); + /* Now we can free the page of PTEs */ free_page((long)ptepage); + /* And zero out the PGD entry we we never release it twice. */ spgd->raw.val = 0; } } +/*H:440 (v) Flushing (thowing away) page tables, + * + * We saw flush_user_mappings() called when we re-used a top-level pgdir page. + * It simply releases every PTE page from 0 up to the kernel address. */ static void flush_user_mappings(struct lguest *lg, int idx) { unsigned int i; + /* Release every pgd entry up to the kernel's address. */ for (i = 0; i < vaddr_to_pgd_index(lg->page_offset); i++) release_pgd(lg, lg->pgdirs[idx].pgdir + i); } +/* The Guest also has a hypercall to do this manually: it's used when a large + * number of mappings have been changed. */ void guest_pagetable_flush_user(struct lguest *lg) { + /* Drop the userspace part of the current page table. */ flush_user_mappings(lg, lg->pgdidx); } +/*:*/ +/* We keep several page tables. This is a simple routine to find the page + * table (if any) corresponding to this top-level address the Guest has given + * us. */ static unsigned int find_pgdir(struct lguest *lg, unsigned long pgtable) { unsigned int i; @@ -232,21 +376,30 @@ static unsigned int find_pgdir(struct lguest *lg, unsigned long pgtable) return i; } +/*H:435 And this is us, creating the new page directory. If we really do + * allocate a new one (and so the kernel parts are not there), we set + * blank_pgdir. */ static unsigned int new_pgdir(struct lguest *lg, unsigned long cr3, int *blank_pgdir) { unsigned int next; + /* We pick one entry at random to throw out. Choosing the Least + * Recently Used might be better, but this is easy. */ next = random32() % ARRAY_SIZE(lg->pgdirs); + /* If it's never been allocated at all before, try now. */ if (!lg->pgdirs[next].pgdir) { lg->pgdirs[next].pgdir = (spgd_t *)get_zeroed_page(GFP_KERNEL); + /* If the allocation fails, just keep using the one we have */ if (!lg->pgdirs[next].pgdir) next = lg->pgdidx; else - /* There are no mappings: you'll need to re-pin */ + /* This is a blank page, so there are no kernel + * mappings: caller must map the stack! */ *blank_pgdir = 1; } + /* Record which Guest toplevel this shadows. */ lg->pgdirs[next].cr3 = cr3; /* Release all the non-kernel mappings. */ flush_user_mappings(lg, next); @@ -254,82 +407,161 @@ static unsigned int new_pgdir(struct lguest *lg, return next; } +/*H:430 (iv) Switching page tables + * + * This is what happens when the Guest changes page tables (ie. changes the + * top-level pgdir). This happens on almost every context switch. */ void guest_new_pagetable(struct lguest *lg, unsigned long pgtable) { int newpgdir, repin = 0; + /* Look to see if we have this one already. */ newpgdir = find_pgdir(lg, pgtable); + /* If not, we allocate or mug an existing one: if it's a fresh one, + * repin gets set to 1. */ if (newpgdir == ARRAY_SIZE(lg->pgdirs)) newpgdir = new_pgdir(lg, pgtable, &repin); + /* Change the current pgd index to the new one. */ lg->pgdidx = newpgdir; + /* If it was completely blank, we map in the Guest kernel stack */ if (repin) pin_stack_pages(lg); } +/*H:470 Finally, a routine which throws away everything: all PGD entries in all + * the shadow page tables. This is used when we destroy the Guest. */ static void release_all_pagetables(struct lguest *lg) { unsigned int i, j; + /* Every shadow pagetable this Guest has */ for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++) if (lg->pgdirs[i].pgdir) + /* Every PGD entry except the Switcher at the top */ for (j = 0; j < SWITCHER_PGD_INDEX; j++) release_pgd(lg, lg->pgdirs[i].pgdir + j); } +/* We also throw away everything when a Guest tells us it's changed a kernel + * mapping. Since kernel mappings are in every page table, it's easiest to + * throw them all away. This is amazingly slow, but thankfully rare. */ void guest_pagetable_clear_all(struct lguest *lg) { release_all_pagetables(lg); + /* We need the Guest kernel stack mapped again. */ pin_stack_pages(lg); } +/*H:420 This is the routine which actually sets the page table entry for then + * "idx"'th shadow page table. + * + * Normally, we can just throw out the old entry and replace it with 0: if they + * use it demand_page() will put the new entry in. We need to do this anyway: + * The Guest expects _PAGE_ACCESSED to be set on its PTE the first time a page + * is read from, and _PAGE_DIRTY when it's written to. + * + * But Avi Kivity pointed out that most Operating Systems (Linux included) set + * these bits on PTEs immediately anyway. This is done to save the CPU from + * having to update them, but it helps us the same way: if they set + * _PAGE_ACCESSED then we can put a read-only PTE entry in immediately, and if + * they set _PAGE_DIRTY then we can put a writable PTE entry in immediately. + */ static void do_set_pte(struct lguest *lg, int idx, unsigned long vaddr, gpte_t gpte) { + /* Look up the matching shadow page directot entry. */ spgd_t *spgd = spgd_addr(lg, idx, vaddr); + + /* If the top level isn't present, there's no entry to update. */ if (spgd->flags & _PAGE_PRESENT) { + /* Otherwise, we start by releasing the existing entry. */ spte_t *spte = spte_addr(lg, *spgd, vaddr); release_pte(*spte); + + /* If they're setting this entry as dirty or accessed, we might + * as well put that entry they've given us in now. This shaves + * 10% off a copy-on-write micro-benchmark. */ if (gpte.flags & (_PAGE_DIRTY | _PAGE_ACCESSED)) { check_gpte(lg, gpte); *spte = gpte_to_spte(lg, gpte, gpte.flags&_PAGE_DIRTY); } else + /* Otherwise we can demand_page() it in later. */ spte->raw.val = 0; } } +/*H:410 Updating a PTE entry is a little trickier. + * + * We keep track of several different page tables (the Guest uses one for each + * process, so it makes sense to cache at least a few). Each of these have + * identical kernel parts: ie. every mapping above PAGE_OFFSET is the same for + * all processes. So when the page table above that address changes, we update + * all the page tables, not just the current one. This is rare. + * + * The benefit is that when we have to track a new page table, we can copy keep + * all the kernel mappings. This speeds up context switch immensely. */ void guest_set_pte(struct lguest *lg, unsigned long cr3, unsigned long vaddr, gpte_t gpte) { - /* Kernel mappings must be changed on all top levels. */ + /* Kernel mappings must be changed on all top levels. Slow, but + * doesn't happen often. */ if (vaddr >= lg->page_offset) { unsigned int i; for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++) if (lg->pgdirs[i].pgdir) do_set_pte(lg, i, vaddr, gpte); } else { + /* Is this page table one we have a shadow for? */ int pgdir = find_pgdir(lg, cr3); if (pgdir != ARRAY_SIZE(lg->pgdirs)) + /* If so, do the update. */ do_set_pte(lg, pgdir, vaddr, gpte); } } +/*H:400 + * (iii) Setting up a page table entry when the Guest tells us it has changed. + * + * Just like we did in interrupts_and_traps.c, it makes sense for us to deal + * with the other side of page tables while we're here: what happens when the + * Guest asks for a page table to be updated? + * + * We already saw that demand_page() will fill in the shadow page tables when + * needed, so we can simply remove shadow page table entries whenever the Guest + * tells us they've changed. When the Guest tries to use the new entry it will + * fault and demand_page() will fix it up. + * + * So with that in mind here's our code to to update a (top-level) PGD entry: + */ void guest_set_pmd(struct lguest *lg, unsigned long cr3, u32 idx) { int pgdir; + /* The kernel seems to try to initialize this early on: we ignore its + * attempts to map over the Switcher. */ if (idx >= SWITCHER_PGD_INDEX) return; + /* If they're talking about a page table we have a shadow for... */ pgdir = find_pgdir(lg, cr3); if (pgdir < ARRAY_SIZE(lg->pgdirs)) + /* ... throw it away. */ release_pgd(lg, lg->pgdirs[pgdir].pgdir + idx); } +/*H:500 (vii) Setting up the page tables initially. + * + * When a Guest is first created, the Launcher tells us where the toplevel of + * its first page table is. We set some things up here: */ int init_guest_pagetable(struct lguest *lg, unsigned long pgtable) { - /* We assume this in flush_user_mappings, so check now */ + /* In flush_user_mappings() we loop from 0 to + * "vaddr_to_pgd_index(lg->page_offset)". This assumes it won't hit + * the Switcher mappings, so check that now. */ if (vaddr_to_pgd_index(lg->page_offset) >= SWITCHER_PGD_INDEX) return -EINVAL; + /* We start on the first shadow page table, and give it a blank PGD + * page. */ lg->pgdidx = 0; lg->pgdirs[lg->pgdidx].cr3 = pgtable; lg->pgdirs[lg->pgdidx].pgdir = (spgd_t*)get_zeroed_page(GFP_KERNEL); @@ -338,33 +570,48 @@ int init_guest_pagetable(struct lguest *lg, unsigned long pgtable) return 0; } +/* When a Guest dies, our cleanup is fairly simple. */ void free_guest_pagetable(struct lguest *lg) { unsigned int i; + /* Throw away all page table pages. */ release_all_pagetables(lg); + /* Now free the top levels: free_page() can handle 0 just fine. */ for (i = 0; i < ARRAY_SIZE(lg->pgdirs); i++) free_page((long)lg->pgdirs[i].pgdir); } -/* Caller must be preempt-safe */ +/*H:480 (vi) Mapping the Switcher when the Guest is about to run. + * + * The Switcher and the two pages for this CPU need to be available to the + * Guest (and not the pages for other CPUs). We have the appropriate PTE pages + * for each CPU already set up, we just need to hook them in. */ void map_switcher_in_guest(struct lguest *lg, struct lguest_pages *pages) { spte_t *switcher_pte_page = __get_cpu_var(switcher_pte_pages); spgd_t switcher_pgd; spte_t regs_pte; - /* Since switcher less that 4MB, we simply mug top pte page. */ + /* Make the last PGD entry for this Guest point to the Switcher's PTE + * page for this CPU (with appropriate flags). */ switcher_pgd.pfn = __pa(switcher_pte_page) >> PAGE_SHIFT; switcher_pgd.flags = _PAGE_KERNEL; lg->pgdirs[lg->pgdidx].pgdir[SWITCHER_PGD_INDEX] = switcher_pgd; - /* Map our regs page over stack page. */ + /* We also change the Switcher PTE page. When we're running the Guest, + * we want the Guest's "regs" page to appear where the first Switcher + * page for this CPU is. This is an optimization: when the Switcher + * saves the Guest registers, it saves them into the first page of this + * CPU's "struct lguest_pages": if we make sure the Guest's register + * page is already mapped there, we don't have to copy them out + * again. */ regs_pte.pfn = __pa(lg->regs_page) >> PAGE_SHIFT; regs_pte.flags = _PAGE_KERNEL; switcher_pte_page[(unsigned long)pages/PAGE_SIZE%PTES_PER_PAGE] = regs_pte; } +/*:*/ static void free_switcher_pte_pages(void) { @@ -374,6 +621,10 @@ static void free_switcher_pte_pages(void) free_page((long)switcher_pte_page(i)); } +/*H:520 Setting up the Switcher PTE page for given CPU is fairly easy, given + * the CPU number and the "struct page"s for the Switcher code itself. + * + * Currently the Switcher is less than a page long, so "pages" is always 1. */ static __init void populate_switcher_pte_page(unsigned int cpu, struct page *switcher_page[], unsigned int pages) @@ -381,21 +632,26 @@ static __init void populate_switcher_pte_page(unsigned int cpu, unsigned int i; spte_t *pte = switcher_pte_page(cpu); + /* The first entries are easy: they map the Switcher code. */ for (i = 0; i < pages; i++) { pte[i].pfn = page_to_pfn(switcher_page[i]); pte[i].flags = _PAGE_PRESENT|_PAGE_ACCESSED; } - /* We only map this CPU's pages, so guest can't see others. */ + /* The only other thing we map is this CPU's pair of pages. */ i = pages + cpu*2; - /* First page (regs) is rw, second (state) is ro. */ + /* First page (Guest registers) is writable from the Guest */ pte[i].pfn = page_to_pfn(switcher_page[i]); pte[i].flags = _PAGE_PRESENT|_PAGE_ACCESSED|_PAGE_RW; + /* The second page contains the "struct lguest_ro_state", and is + * read-only. */ pte[i+1].pfn = page_to_pfn(switcher_page[i+1]); pte[i+1].flags = _PAGE_PRESENT|_PAGE_ACCESSED; } +/*H:510 At boot or module load time, init_pagetables() allocates and populates + * the Switcher PTE page for each CPU. */ __init int init_pagetables(struct page **switcher_page, unsigned int pages) { unsigned int i; @@ -410,7 +666,9 @@ __init int init_pagetables(struct page **switcher_page, unsigned int pages) } return 0; } +/*:*/ +/* Cleaning up simply involves freeing the PTE page for each CPU. */ void free_pagetables(void) { free_switcher_pte_pages(); diff --git a/drivers/lguest/segments.c b/drivers/lguest/segments.c index c4fc729..4d4e5a4 100644 --- a/drivers/lguest/segments.c +++ b/drivers/lguest/segments.c @@ -11,17 +11,58 @@ * from frolicking through its parklike serenity. :*/ #include "lg.h" +/*H:600 + * We've almost completed the Host; there's just one file to go! + * + * Segments & The Global Descriptor Table + * + * (That title sounds like a bad Nerdcore group. Not to suggest that there are + * any good Nerdcore groups, but in high school a friend of mine had a band + * called Joe Fish and the Chips, so there are definitely worse band names). + * + * To refresh: the GDT is a table of 8-byte values describing segments. Once + * set up, these segments can be loaded into one of the 6 "segment registers". + * + * GDT entries are passed around as "struct desc_struct"s, which like IDT + * entries are split into two 32-bit members, "a" and "b". One day, someone + * will clean that up, and be declared a Hero. (No pressure, I'm just saying). + * + * Anyway, the GDT entry contains a base (the start address of the segment), a + * limit (the size of the segment - 1), and some flags. Sounds simple, and it + * would be, except those zany Intel engineers decided that it was too boring + * to put the base at one end, the limit at the other, and the flags in + * between. They decided to shotgun the bits at random throughout the 8 bytes, + * like so: + * + * 0 16 40 48 52 56 63 + * [ limit part 1 ][ base part 1 ][ flags ][li][fl][base ] + * mit ags part 2 + * part 2 + * + * As a result, this file contains a certain amount of magic numeracy. Let's + * begin. + */ + +/* Is the descriptor the Guest wants us to put in OK? + * + * The flag which Intel says must be zero: must be zero. The descriptor must + * be present, (this is actually checked earlier but is here for thorougness), + * and the descriptor type must be 1 (a memory segment). */ static int desc_ok(const struct desc_struct *gdt) { - /* MBZ=0, P=1, DT=1 */ return ((gdt->b & 0x00209000) == 0x00009000); } +/* Is the segment present? (Otherwise it can't be used by the Guest). */ static int segment_present(const struct desc_struct *gdt) { return gdt->b & 0x8000; } +/* There are several entries we don't let the Guest set. The TSS entry is the + * "Task State Segment" which controls all kinds of delicate things. The + * LGUEST_CS and LGUEST_DS entries are reserved for the Switcher, and the + * the Guest can't be trusted to deal with double faults. */ static int ignored_gdt(unsigned int num) { return (num == GDT_ENTRY_TSS @@ -30,9 +71,18 @@ static int ignored_gdt(unsigned int num) || num == GDT_ENTRY_DOUBLEFAULT_TSS); } -/* We don't allow removal of CS, DS or SS; it doesn't make sense. */ +/* If the Guest asks us to remove an entry from the GDT, we have to be careful. + * If one of the segment registers is pointing at that entry the Switcher will + * crash when it tries to reload the segment registers for the Guest. + * + * It doesn't make much sense for the Guest to try to remove its own code, data + * or stack segments while they're in use: assume that's a Guest bug. If it's + * one of the lesser segment registers using the removed entry, we simply set + * that register to 0 (unusable). */ static void check_segment_use(struct lguest *lg, unsigned int desc) { + /* GDT entries are 8 bytes long, so we divide to get the index and + * ignore the bottom bits. */ if (lg->regs->gs / 8 == desc) lg->regs->gs = 0; if (lg->regs->fs / 8 == desc) @@ -45,12 +95,16 @@ static void check_segment_use(struct lguest *lg, unsigned int desc) kill_guest(lg, "Removed live GDT entry %u", desc); } +/*H:610 Once the GDT has been changed, we look through the changed entries and + * see if they're OK. If not, we'll call kill_guest() and the Guest will never + * get to use the invalid entries. */ static void fixup_gdt_table(struct lguest *lg, unsigned start, unsigned end) { unsigned int i; for (i = start; i < end; i++) { - /* We never copy these ones to real gdt */ + /* We never copy these ones to real GDT, so we don't care what + * they say */ if (ignored_gdt(i)) continue; @@ -64,41 +118,57 @@ static void fixup_gdt_table(struct lguest *lg, unsigned start, unsigned end) if (!desc_ok(&lg->gdt[i])) kill_guest(lg, "Bad GDT descriptor %i", i); - /* DPL 0 presumably means "for use by guest". */ + /* Segment descriptors contain a privilege level: the Guest is + * sometimes careless and leaves this as 0, even though it's + * running at privilege level 1. If so, we fix it here. */ if ((lg->gdt[i].b & 0x00006000) == 0) lg->gdt[i].b |= (GUEST_PL << 13); - /* Set accessed bit, since gdt isn't writable. */ + /* Each descriptor has an "accessed" bit. If we don't set it + * now, the CPU will try to set it when the Guest first loads + * that entry into a segment register. But the GDT isn't + * writable by the Guest, so bad things can happen. */ lg->gdt[i].b |= 0x00000100; } } +/* This routine is called at boot or modprobe time for each CPU to set up the + * "constant" GDT entries for Guests running on that CPU. */ void setup_default_gdt_entries(struct lguest_ro_state *state) { struct desc_struct *gdt = state->guest_gdt; unsigned long tss = (unsigned long)&state->guest_tss; - /* Hypervisor segments. */ + /* The hypervisor segments are full 0-4G segments, privilege level 0 */ gdt[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT; gdt[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT; - /* This is the one which we *cannot* copy from guest, since tss - is depended on this lguest_ro_state, ie. this cpu. */ + /* The TSS segment refers to the TSS entry for this CPU, so we cannot + * copy it from the Guest. Forgive the magic flags */ gdt[GDT_ENTRY_TSS].a = 0x00000067 | (tss << 16); gdt[GDT_ENTRY_TSS].b = 0x00008900 | (tss & 0xFF000000) | ((tss >> 16) & 0x000000FF); } +/* This routine is called before the Guest is run for the first time. */ void setup_guest_gdt(struct lguest *lg) { + /* Start with full 0-4G segments... */ lg->gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT; lg->gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT; + /* ...except the Guest is allowed to use them, so set the privilege + * level appropriately in the flags. */ lg->gdt[GDT_ENTRY_KERNEL_CS].b |= (GUEST_PL << 13); lg->gdt[GDT_ENTRY_KERNEL_DS].b |= (GUEST_PL << 13); } -/* This is a fast version for the common case where only the three TLS entries - * have changed. */ +/* Like the IDT, we never simply use the GDT the Guest gives us. We set up the + * GDTs for each CPU, then we copy across the entries each time we want to run + * a different Guest on that CPU. */ + +/* A partial GDT load, for the three "thead-local storage" entries. Otherwise + * it's just like load_guest_gdt(). So much, in fact, it would probably be + * neater to have a single hypercall to cover both. */ void copy_gdt_tls(const struct lguest *lg, struct desc_struct *gdt) { unsigned int i; @@ -107,22 +177,31 @@ void copy_gdt_tls(const struct lguest *lg, struct desc_struct *gdt) gdt[i] = lg->gdt[i]; } +/* This is the full version */ void copy_gdt(const struct lguest *lg, struct desc_struct *gdt) { unsigned int i; + /* The default entries from setup_default_gdt_entries() are not + * replaced. See ignored_gdt() above. */ for (i = 0; i < GDT_ENTRIES; i++) if (!ignored_gdt(i)) gdt[i] = lg->gdt[i]; } +/* This is where the Guest asks us to load a new GDT (LHCALL_LOAD_GDT). */ void load_guest_gdt(struct lguest *lg, unsigned long table, u32 num) { + /* We assume the Guest has the same number of GDT entries as the + * Host, otherwise we'd have to dynamically allocate the Guest GDT. */ if (num > ARRAY_SIZE(lg->gdt)) kill_guest(lg, "too many gdt entries %i", num); + /* We read the whole thing in, then fix it up. */ lgread(lg, lg->gdt, table, num * sizeof(lg->gdt[0])); fixup_gdt_table(lg, 0, ARRAY_SIZE(lg->gdt)); + /* Mark that the GDT changed so the core knows it has to copy it again, + * even if the Guest is run on the same CPU. */ lg->changed |= CHANGED_GDT; } @@ -134,3 +213,13 @@ void guest_load_tls(struct lguest *lg, unsigned long gtls) fixup_gdt_table(lg, GDT_ENTRY_TLS_MIN, GDT_ENTRY_TLS_MAX+1); lg->changed |= CHANGED_GDT_TLS; } + +/* + * With this, we have finished the Host. + * + * Five of the seven parts of our task are complete. You have made it through + * the Bit of Despair (I think that's somewhere in the page table code, + * myself). + * + * Next, we examine "make Switcher". It's short, but intense. + */ |
