| #include <linux/linkage.h> |
| #include <linux/lguest.h> |
| #include <asm/asm-offsets.h> |
| #include <asm/thread_info.h> |
| #include <asm/processor-flags.h> |
| |
| /*G:020 This is where we begin: we have a magic signature which the launcher |
| * looks for. The plan is that the Linux boot protocol will be extended with a |
| * "platform type" field which will guide us here from the normal entry point, |
| * but for the moment this suffices. The normal boot code uses %esi for the |
| * boot header, so we do too. We convert it to a virtual address by adding |
| * PAGE_OFFSET, and hand it to lguest_init() as its argument (ie. %eax). |
| * |
| * The .section line puts this code in .init.text so it will be discarded after |
| * boot. */ |
| .section .init.text, "ax", @progbits |
| .ascii "GenuineLguest" |
| /* Set up initial stack. */ |
| movl $(init_thread_union+THREAD_SIZE),%esp |
| movl %esi, %eax |
| addl $__PAGE_OFFSET, %eax |
| jmp lguest_init |
| |
| /*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(sti, movl $X86_EFLAGS_IF, lguest_data+LGUEST_DATA_irq_enabled) |
| LGUEST_PATCH(popf, movl %eax, lguest_data+LGUEST_DATA_irq_enabled) |
| LGUEST_PATCH(pushf, movl lguest_data+LGUEST_DATA_irq_enabled, %eax) |
| /*:*/ |
| |
| /* These demark the EIP range where host should never deliver interrupts. */ |
| .global lguest_noirq_start |
| .global lguest_noirq_end |
| |
| /*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 need to use a register to do |
| * the copy and secondly, the whole thing needs to be atomic. The first |
| * problem is easy to solve: push %eax on the stack so we can use it, and then |
| * restore it at the end just before the real "iret". |
| * |
| * 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 surround the |
| * code with lguest_noirq_start: and lguest_noirq_end: labels. We tell the |
| * Host that it is *never* to interrupt us there, even if interrupts seem to be |
| * enabled. */ |
| ENTRY(lguest_iret) |
| pushl %eax |
| movl 12(%esp), %eax |
| lguest_noirq_start: |
| /* 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. */ |
| movl %eax,%ss:lguest_data+LGUEST_DATA_irq_enabled |
| popl %eax |
| iret |
| lguest_noirq_end: |