diff options
Diffstat (limited to 'arch/x86/lguest/head_32.S')
| -rw-r--r-- | arch/x86/lguest/head_32.S | 192 |
1 files changed, 0 insertions, 192 deletions
diff --git a/arch/x86/lguest/head_32.S b/arch/x86/lguest/head_32.S deleted file mode 100644 index d5ae63f..0000000 --- a/arch/x86/lguest/head_32.S +++ /dev/null @@ -1,192 +0,0 @@ -#include <linux/linkage.h> -#include <linux/lguest.h> -#include <asm/lguest_hcall.h> -#include <asm/asm-offsets.h> -#include <asm/thread_info.h> -#include <asm/processor-flags.h> - -/*G:020 - - * Our story starts with the bzImage: booting starts at startup_32 in - * arch/x86/boot/compressed/head_32.S. This merely uncompresses the real - * kernel in place and then jumps into it: startup_32 in - * arch/x86/kernel/head_32.S. Both routines expects a boot header in the %esi - * register, which is created by the bootloader (the Launcher in our case). - * - * The startup_32 function does very little: it clears the uninitialized global - * C variables which we expect to be zero (ie. BSS) and then copies the boot - * header and kernel command line somewhere safe, and populates some initial - * page tables. Finally it checks the 'hardware_subarch' field. This was - * introduced in 2.6.24 for lguest and Xen: if it's set to '1' (lguest's - * assigned number), then it calls us here. - * - * WARNING: be very careful here! We're running at addresses equal to physical - * addresses (around 0), not above PAGE_OFFSET as most code expects - * (eg. 0xC0000000). Jumps are relative, so they're OK, but we can't touch any - * data without remembering to subtract __PAGE_OFFSET! - * - * The .section line puts this code in .init.text so it will be discarded after - * boot. - */ -.section .init.text, "ax", @progbits -ENTRY(lguest_entry) - /* - * We make the "initialization" hypercall now to tell the Host where - * our lguest_data struct is. - */ - movl $LHCALL_LGUEST_INIT, %eax - movl $lguest_data - __PAGE_OFFSET, %ebx - int $LGUEST_TRAP_ENTRY - - /* Now turn our pagetables on; setup by arch/x86/kernel/head_32.S. */ - movl $LHCALL_NEW_PGTABLE, %eax - movl $(initial_page_table - __PAGE_OFFSET), %ebx - int $LGUEST_TRAP_ENTRY - - /* Set up the initial stack so we can run C code. */ - movl $(init_thread_union+THREAD_SIZE),%esp - - /* Jumps are relative: we're running __PAGE_OFFSET too low. */ - jmp lguest_init+__PAGE_OFFSET - -/*G:055 - * We create a macro which puts the assembler code between lgstart_ and lgend_ - * markers. These templates are put in the .text section: they can't be - * discarded after boot as we may need to patch modules, too. - */ -.text -#define LGUEST_PATCH(name, insns...) \ - lgstart_##name: insns; lgend_##name:; \ - .globl lgstart_##name; .globl lgend_##name - -LGUEST_PATCH(cli, movl $0, lguest_data+LGUEST_DATA_irq_enabled) -LGUEST_PATCH(pushf, movl lguest_data+LGUEST_DATA_irq_enabled, %eax) - -/*G:033 - * But using those wrappers is inefficient (we'll see why that doesn't matter - * for save_fl and irq_disable later). If we write our routines carefully in - * assembler, we can avoid clobbering any registers and avoid jumping through - * the wrapper functions. - * - * I skipped over our first piece of assembler, but this one is worth studying - * in a bit more detail so I'll describe in easy stages. First, the routine to - * enable interrupts: - */ -ENTRY(lg_irq_enable) - /* - * The reverse of irq_disable, this sets lguest_data.irq_enabled to - * X86_EFLAGS_IF (ie. "Interrupts enabled"). - */ - movl $X86_EFLAGS_IF, lguest_data+LGUEST_DATA_irq_enabled - /* - * But now we need to check if the Host wants to know: there might have - * been interrupts waiting to be delivered, in which case it will have - * set lguest_data.irq_pending to X86_EFLAGS_IF. If it's not zero, we - * jump to send_interrupts, otherwise we're done. - */ - cmpl $0, lguest_data+LGUEST_DATA_irq_pending - jnz send_interrupts - /* - * One cool thing about x86 is that you can do many things without using - * a register. In this case, the normal path hasn't needed to save or - * restore any registers at all! - */ - ret -send_interrupts: - /* - * OK, now we need a register: eax is used for the hypercall number, - * which is LHCALL_SEND_INTERRUPTS. - * - * We used not to bother with this pending detection at all, which was - * much simpler. Sooner or later the Host would realize it had to - * send us an interrupt. But that turns out to make performance 7 - * times worse on a simple tcp benchmark. So now we do this the hard - * way. - */ - pushl %eax - movl $LHCALL_SEND_INTERRUPTS, %eax - /* This is the actual hypercall trap. */ - int $LGUEST_TRAP_ENTRY - /* Put eax back the way we found it. */ - popl %eax - ret - -/* - * Finally, the "popf" or "restore flags" routine. The %eax register holds the - * flags (in practice, either X86_EFLAGS_IF or 0): if it's X86_EFLAGS_IF we're - * enabling interrupts again, if it's 0 we're leaving them off. - */ -ENTRY(lg_restore_fl) - /* This is just "lguest_data.irq_enabled = flags;" */ - movl %eax, lguest_data+LGUEST_DATA_irq_enabled - /* - * Now, if the %eax value has enabled interrupts and - * lguest_data.irq_pending is set, we want to tell the Host so it can - * deliver any outstanding interrupts. Fortunately, both values will - * be X86_EFLAGS_IF (ie. 512) in that case, and the "testl" - * instruction will AND them together for us. If both are set, we - * jump to send_interrupts. - */ - testl lguest_data+LGUEST_DATA_irq_pending, %eax - jnz send_interrupts - /* Again, the normal path has used no extra registers. Clever, huh? */ - ret -/*:*/ - -/* These demark the EIP where host should never deliver interrupts. */ -.global lguest_noirq_iret - -/*M:004 - * When the Host reflects a trap or injects an interrupt into the Guest, it - * sets the eflags interrupt bit on the stack based on lguest_data.irq_enabled, - * so the Guest iret logic does the right thing when restoring it. However, - * when the Host sets the Guest up for direct traps, such as system calls, the - * processor is the one to push eflags onto the stack, and the interrupt bit - * will be 1 (in reality, interrupts are always enabled in the Guest). - * - * This turns out to be harmless: the only trap which should happen under Linux - * with interrupts disabled is Page Fault (due to our lazy mapping of vmalloc - * regions), which has to be reflected through the Host anyway. If another - * trap *does* go off when interrupts are disabled, the Guest will panic, and - * we'll never get to this iret! -:*/ - -/*G:045 - * There is one final paravirt_op that the Guest implements, and glancing at it - * you can see why I left it to last. It's *cool*! It's in *assembler*! - * - * The "iret" instruction is used to return from an interrupt or trap. The - * stack looks like this: - * old address - * old code segment & privilege level - * old processor flags ("eflags") - * - * The "iret" instruction pops those values off the stack and restores them all - * at once. The only problem is that eflags includes the Interrupt Flag which - * the Guest can't change: the CPU will simply ignore it when we do an "iret". - * So we have to copy eflags from the stack to lguest_data.irq_enabled before - * we do the "iret". - * - * There are two problems with this: firstly, we can't clobber any registers - * and secondly, the whole thing needs to be atomic. The first problem - * is solved by using "push memory"/"pop memory" instruction pair for copying. - * - * The second is harder: copying eflags to lguest_data.irq_enabled will turn - * interrupts on before we're finished, so we could be interrupted before we - * return to userspace or wherever. Our solution to this is to tell the - * Host that it is *never* to interrupt us there, even if interrupts seem to be - * enabled. (It's not necessary to protect pop instruction, since - * data gets updated only after it completes, so we only need to protect - * one instruction, iret). - */ -ENTRY(lguest_iret) - pushl 2*4(%esp) - /* - * Note the %ss: segment prefix here. Normal data accesses use the - * "ds" segment, but that will have already been restored for whatever - * we're returning to (such as userspace): we can't trust it. The %ss: - * prefix makes sure we use the stack segment, which is still valid. - */ - popl %ss:lguest_data+LGUEST_DATA_irq_enabled -lguest_noirq_iret: - iret |
