diff options
Diffstat (limited to 'arch/x86/lguest/head_32.S')
| -rw-r--r-- | arch/x86/lguest/head_32.S | 192 |
1 files changed, 192 insertions, 0 deletions
diff --git a/arch/x86/lguest/head_32.S b/arch/x86/lguest/head_32.S new file mode 100644 index 000000000..d5ae63f5e --- /dev/null +++ b/arch/x86/lguest/head_32.S @@ -0,0 +1,192 @@ +#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 |