diff options
Diffstat (limited to 'drivers/lguest/x86')
| -rw-r--r-- | drivers/lguest/x86/core.c | 120 | ||||
| -rw-r--r-- | drivers/lguest/x86/switcher_32.S | 71 |
2 files changed, 113 insertions, 78 deletions
diff --git a/drivers/lguest/x86/core.c b/drivers/lguest/x86/core.c index 09d9207420dc..482aec2a9631 100644 --- a/drivers/lguest/x86/core.c +++ b/drivers/lguest/x86/core.c @@ -63,7 +63,7 @@ static struct lguest_pages *lguest_pages(unsigned int cpu) static DEFINE_PER_CPU(struct lguest *, last_guest); /*S:010 - * We are getting close to the Switcher. + * We approach the Switcher. * * Remember that each CPU has two pages which are visible to the Guest when it * runs on that CPU. This has to contain the state for that Guest: we copy the @@ -134,7 +134,7 @@ static void run_guest_once(struct lguest *lg, struct lguest_pages *pages) * * The lcall also pushes the old code segment (KERNEL_CS) onto the * stack, then the address of this call. This stack layout happens to - * exactly match the stack of an interrupt... */ + * exactly match the stack layout created by an interrupt... */ asm volatile("pushf; lcall *lguest_entry" /* This is how we tell GCC that %eax ("a") and %ebx ("b") * are changed by this routine. The "=" means output. */ @@ -151,40 +151,46 @@ static void run_guest_once(struct lguest *lg, struct lguest_pages *pages) } /*:*/ +/*M:002 There are hooks in the scheduler which we can register to tell when we + * get kicked off the CPU (preempt_notifier_register()). This would allow us + * to lazily disable SYSENTER which would regain some performance, and should + * also simplify copy_in_guest_info(). Note that we'd still need to restore + * things when we exit to Launcher userspace, but that's fairly easy. + * + * The hooks were designed for KVM, but we can also put them to good use. :*/ + /*H:040 This is the i386-specific code to setup and run the Guest. Interrupts * are disabled: we own the CPU. */ void lguest_arch_run_guest(struct lguest *lg) { - /* 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. */ + /* 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) lguest_set_ts(); - /* 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. */ + /* 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. */ + /* Now we actually run the Guest. It will return 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())); - /* 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. */ + /* Note that 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 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) lg->arch.last_pagefault = read_cr2(); /* Similarly, if we took a trap because the Guest used the FPU, @@ -197,14 +203,15 @@ void lguest_arch_run_guest(struct lguest *lg) wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0); } -/*H:130 Our Guest is usually so well behaved; it never tries to do things it - * isn't allowed to. Unfortunately, Linux's paravirtual infrastructure 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. +/*H:130 Now we've examined the hypercall code; our Guest can make requests. + * Our Guest is usually so well behaved; it never tries to do things it isn't + * allowed to, and uses hypercalls instead. Unfortunately, Linux's paravirtual + * infrastructure 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 + * When the Guest uses one of these instructions, we get a trap (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) @@ -275,43 +282,43 @@ static int emulate_insn(struct lguest *lg) void lguest_arch_handle_trap(struct lguest *lg) { 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. */ + case 13: /* We've intercepted a General Protection Fault. */ + /* 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)) return; } 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. */ + 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, lg->arch.last_pagefault, lg->regs->errcode)) return; - /* 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 */ + /* 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 LHCALL_LGUEST_INIT hypercall, so + * lg->lguest_data could be NULL */ if (lg->lguest_data && put_user(lg->arch.last_pagefault, &lg->lguest_data->cr2)) kill_guest(lg, "Writing cr2"); break; case 7: /* We've intercepted a Device Not Available fault. */ - /* If the Guest doesn't want to know, we already - * restored the Floating Point Unit, so we just - * continue without telling 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) return; break; @@ -536,9 +543,6 @@ int lguest_arch_init_hypercalls(struct lguest *lg) return 0; } -/* 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(). :*/ /*L:030 lguest_arch_setup_regs() * @@ -570,8 +574,8 @@ void lguest_arch_setup_regs(struct lguest *lg, unsigned long start) /* %esi points to our boot information, at physical address 0, so don't * touch it. */ + /* There are a couple of GDT entries the Guest expects when first * booting. */ - setup_guest_gdt(lg); } diff --git a/drivers/lguest/x86/switcher_32.S b/drivers/lguest/x86/switcher_32.S index 1010b90b11fc..0af8baaa0d4a 100644 --- a/drivers/lguest/x86/switcher_32.S +++ b/drivers/lguest/x86/switcher_32.S @@ -6,6 +6,37 @@ * are feeling invigorated and refreshed then the next, more challenging stage * can be found in "make Guest". :*/ +/*M:012 Lguest is meant to be simple: my rule of thumb is that 1% more LOC must + * gain at least 1% more performance. Since neither LOC nor performance can be + * measured beforehand, it generally means implementing a feature then deciding + * if it's worth it. And once it's implemented, who can say no? + * + * This is why I haven't implemented this idea myself. I want to, but I + * haven't. You could, though. + * + * The main place where lguest performance sucks is Guest page faulting. When + * a Guest userspace process hits an unmapped page we switch back to the Host, + * walk the page tables, find it's not mapped, switch back to the Guest page + * fault handler, which calls a hypercall to set the page table entry, then + * finally returns to userspace. That's two round-trips. + * + * If we had a small walker in the Switcher, we could quickly check the Guest + * page table and if the page isn't mapped, immediately reflect the fault back + * into the Guest. This means the Switcher would have to know the top of the + * Guest page table and the page fault handler address. + * + * For simplicity, the Guest should only handle the case where the privilege + * level of the fault is 3 and probably only not present or write faults. It + * should also detect recursive faults, and hand the original fault to the + * Host (which is actually really easy). + * + * Two questions remain. Would the performance gain outweigh the complexity? + * And who would write the verse documenting it? :*/ + +/*M:011 Lguest64 handles NMI. This gave me NMI envy (until I looked at their + * code). It's worth doing though, since it would let us use oprofile in the + * Host when a Guest is running. :*/ + /*S:100 * Welcome to the Switcher itself! * @@ -88,7 +119,7 @@ ENTRY(switch_to_guest) // All saved and there's now five steps before us: // Stack, GDT, IDT, TSS - // And last of all the page tables are flipped. + // Then last of all the page tables are flipped. // Yet beware that our stack pointer must be // Always valid lest an NMI hits @@ -103,25 +134,25 @@ ENTRY(switch_to_guest) lgdt LGUEST_PAGES_guest_gdt_desc(%eax) // The Guest's IDT we did partially - // Move to the "struct lguest_pages" as well. + // Copy to "struct lguest_pages" as well. lidt LGUEST_PAGES_guest_idt_desc(%eax) // The TSS entry which controls traps // Must be loaded up with "ltr" now: + // The GDT entry that TSS uses + // Changes type when we load it: damn Intel! // For after we switch over our page tables - // It (as the rest) will be writable no more. - // (The GDT entry TSS needs - // Changes type when we load it: damn Intel!) + // That entry will be read-only: we'd crash. movl $(GDT_ENTRY_TSS*8), %edx ltr %dx // Look back now, before we take this last step! // The Host's TSS entry was also marked used; - // Let's clear it again, ere we return. + // Let's clear it again for our return. // The GDT descriptor of the Host // Points to the table after two "size" bytes movl (LGUEST_PAGES_host_gdt_desc+2)(%eax), %edx - // Clear the type field of "used" (byte 5, bit 2) + // Clear "used" from type field (byte 5, bit 2) andb $0xFD, (GDT_ENTRY_TSS*8 + 5)(%edx) // Once our page table's switched, the Guest is live! @@ -131,7 +162,7 @@ ENTRY(switch_to_guest) // The page table change did one tricky thing: // The Guest's register page has been mapped - // Writable onto our %esp (stack) -- + // Writable under our %esp (stack) -- // We can simply pop off all Guest regs. popl %eax popl %ebx @@ -152,16 +183,15 @@ ENTRY(switch_to_guest) addl $8, %esp // The last five stack slots hold return address - // And everything needed to change privilege - // Into the Guest privilege level of 1, + // And everything needed to switch privilege + // From Switcher's level 0 to Guest's 1, // And the stack where the Guest had last left it. // Interrupts are turned back on: we are Guest. iret -// There are two paths where we switch to the Host +// We treat two paths to switch back to the Host +// Yet both must save Guest state and restore Host // So we put the routine in a macro. -// We are on our way home, back to the Host -// Interrupted out of the Guest, we come here. #define SWITCH_TO_HOST \ /* We save the Guest state: all registers first \ * Laid out just as "struct lguest_regs" defines */ \ @@ -194,7 +224,7 @@ ENTRY(switch_to_guest) movl %esp, %eax; \ andl $(~(1 << PAGE_SHIFT - 1)), %eax; \ /* Save our trap number: the switch will obscure it \ - * (The Guest regs are not mapped here in the Host) \ + * (In the Host the Guest regs are not mapped here) \ * %ebx holds it safe for deliver_to_host */ \ movl LGUEST_PAGES_regs_trapnum(%eax), %ebx; \ /* The Host GDT, IDT and stack! \ @@ -210,9 +240,9 @@ ENTRY(switch_to_guest) /* Switch to Host's GDT, IDT. */ \ lgdt LGUEST_PAGES_host_gdt_desc(%eax); \ lidt LGUEST_PAGES_host_idt_desc(%eax); \ - /* Restore the Host's stack where it's saved regs lie */ \ + /* Restore the Host's stack where its saved regs lie */ \ movl LGUEST_PAGES_host_sp(%eax), %esp; \ - /* Last the TSS: our Host is complete */ \ + /* Last the TSS: our Host is returned */ \ movl $(GDT_ENTRY_TSS*8), %edx; \ ltr %dx; \ /* Restore now the regs saved right at the first. */ \ @@ -222,14 +252,15 @@ ENTRY(switch_to_guest) popl %ds; \ popl %es -// Here's where we come when the Guest has just trapped: -// (Which trap we'll see has been pushed on the stack). +// The first path is trod when the Guest has trapped: +// (Which trap it was has been pushed on the stack). // We need only switch back, and the Host will decode // Why we came home, and what needs to be done. return_to_host: SWITCH_TO_HOST iret +// We are lead to the second path like so: // An interrupt, with some cause external // Has ajerked us rudely from the Guest's code // Again we must return home to the Host @@ -238,7 +269,7 @@ deliver_to_host: // But now we must go home via that place // Where that interrupt was supposed to go // Had we not been ensconced, running the Guest. - // Here we see the cleverness of our stack: + // Here we see the trickness of run_guest_once(): // The Host stack is formed like an interrupt // With EIP, CS and EFLAGS layered. // Interrupt handlers end with "iret" @@ -263,7 +294,7 @@ deliver_to_host: xorw %ax, %ax orl %eax, %edx // Now the address of the handler's in %edx - // We call it now: its "iret" takes us home. + // We call it now: its "iret" drops us home. jmp *%edx // Every interrupt can come to us here |