diff options
Diffstat (limited to 'drivers/lguest/interrupts_and_traps.c')
| -rw-r--r-- | drivers/lguest/interrupts_and_traps.c | 176 |
1 files changed, 157 insertions, 19 deletions
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; |
