/*P:800 Interrupts (traps) are complicated enough to earn their own file. * There are three classes of interrupts: * * 1) Real hardware interrupts which occur while we're running the Guest, * 2) Interrupts for virtual devices attached to the Guest, and * 3) Traps and faults from the Guest. * * Real hardware interrupts must be delivered to the Host, not the Guest. * Virtual interrupts must be delivered to the Guest, but we make them look * just like real hardware would deliver them. Traps from the Guest can be set * up to go directly back into the Guest, but sometimes the Host wants to see * them first, so we also have a way of "reflecting" them into the Guest as if * they had been delivered to it directly. :*/ #include #include #include #include "lg.h" /* Allow Guests to use a non-128 (ie. non-Linux) syscall trap. */ static unsigned int syscall_vector = SYSCALL_VECTOR; module_param(syscall_vector, uint, 0444); /* 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(lg, *gstack, u32, 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, origstack; u32 eflags, ss, irq_enable; unsigned long virtstack; /* 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 */ virtstack = lg->esp1; ss = lg->ss1; origstack = gstack = guest_pa(lg, virtstack); /* 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. */ virtstack = lg->regs->esp; ss = lg->regs->ss; origstack = gstack = guest_pa(lg, virtstack); } /* 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: we saw the Guest * copy 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); /* 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 = virtstack + (gstack - origstack); lg->regs->cs = (__KERNEL_CS|GUEST_PL); lg->regs->eip = idt_address(lo, hi); /* 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:205 * 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; /* 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, 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 { /* Otherwise we check if they have interrupts disabled. */ u32 irq_enabled; if (get_user(irq_enabled, &lg->lguest_data->irq_enabled)) irq_enabled = 0; if (!irq_enabled) 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->arch.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); } /* Every time we deliver an interrupt, we update the timestamp in the * Guest's lguest_data struct. It would be better for the Guest if we * did this more often, but it can actually be quite slow: doing it * here is a compromise which means at least it gets updated every * timer interrupt. */ write_timestamp(lg); } /*:*/ /* Linux uses trap 128 for system calls. Plan9 uses 64, and Ron Minnich sent * me a patch, so we support that too. It'd be a big step for lguest if half * the Plan 9 user base were to start using it. * * Actually now I think of it, it's possible that Ron *is* half the Plan 9 * userbase. Oh well. */ static bool could_be_syscall(unsigned int num) { /* Normal Linux SYSCALL_VECTOR or reserved vector? */ return num == SYSCALL_VECTOR || num == syscall_vector; } /* The syscall vector it wants must be unused by Host. */ bool check_syscall_vector(struct lguest *lg) { u32 vector; if (get_user(vector, &lg->lguest_data->syscall_vec)) return false; return could_be_syscall(vector); } int init_interrupts(void) { /* If they want some strange system call vector, reserve it now */ if (syscall_vector != SYSCALL_VECTOR && test_and_set_bit(syscall_vector, used_vectors)) { printk("lg: couldn't reserve syscall %u\n", syscall_vector); return -EBUSY; } return 0; } void free_interrupts(void) { if (syscall_vector != SYSCALL_VECTOR) clear_bit(syscall_vector, used_vectors); } /*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) { /* Trap numbers are always 8 bit, but we set an impossible trap number * for traps inside the Switcher, so check that here. */ if (num >= ARRAY_SIZE(lg->arch.idt)) return 0; /* 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(lg->arch.idt[num].a, lg->arch.idt[num].b)) return 0; set_guest_interrupt(lg, lg->arch.idt[num].a, lg->arch.idt[num].b, 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 (usually 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 beat it up at lunchtime. * * This routine indicates if a particular trap number could be delivered * directly. */ static int direct_trap(unsigned int num) { /* Hardware interrupts don't go to the Guest at all (except system * call). */ if (num >= FIRST_EXTERNAL_VECTOR && !could_be_syscall(num)) return 0; /* 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. */ return num != 14 && num != 13 && num != 7 && num != LGUEST_TRAP_ENTRY; } /*:*/ /*M:005 The Guest has the ability to turn its interrupt gates into trap gates, * if it is careful. The Host will let trap gates can go directly to the * Guest, but the Guest needs the interrupts atomically disabled for an * interrupt gate. It can do this by pointing the trap gate at instructions * within noirq_start and noirq_end, where it can safely disable interrupts. */ /*M:006 The Guests do not use the sysenter (fast system call) instruction, * because it's hardcoded to enter privilege level 0 and so can't go direct. * It's about twice as fast as the older "int 0x80" system call, so it might * still be worthwhile to handle it in the Switcher and lcall down to the * Guest. The sysenter semantics are hairy tho: search for that keyword in * entry.S :*/ /*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*, so the address we're given is the * start of the page after the kernel stack. Subtract one to * get back onto the first stack page, and keep subtracting to * get to the rest of the stack pages. */ pin_page(lg, lg->esp1 - 1 - 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 are not allowed 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); } /* 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 the 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, 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; /* Check that the Guest doesn't try to step outside the bounds. */ if (num >= ARRAY_SIZE(lg->arch.idt)) kill_guest(lg, "Setting idt entry %u", num); else set_trap(lg, &lg->arch.idt[num], 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; /* 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) { unsigned int i; for (i = 0; i < ARRAY_SIZE(state->guest_idt); i++) 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; /* 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 < ARRAY_SIZE(lg->arch.idt); i++) { /* If no Guest can ever override this trap, leave it alone. */ if (!direct_trap(i)) continue; /* 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. */ if (idt_type(lg->arch.idt[i].a, lg->arch.idt[i].b) == 0xF) idt[i] = lg->arch.idt[i]; else /* Reset it to the default. */ default_idt_entry(&idt[i], i, def[i]); } } /*H:200 * The Guest Clock. * * There are two sources of virtual interrupts. We saw one in lguest_user.c: * the Launcher sending interrupts for virtual devices. The other is the Guest * timer interrupt. * * The Guest uses the LHCALL_SET_CLOCKEVENT hypercall to tell us how long to * the next timer interrupt (in nanoseconds). We use the high-resolution timer * infrastructure to set a callback at that time. * * 0 means "turn off the clock". */ void guest_set_clockevent(struct lg_cpu *cpu, unsigned long delta) { ktime_t expires; if (unlikely(delta == 0)) { /* Clock event device is shutting down. */ hrtimer_cancel(&cpu->hrt); return; } /* We use wallclock time here, so the Guest might not be running for * all the time between now and the timer interrupt it asked for. This * is almost always the right thing to do. */ expires = ktime_add_ns(ktime_get_real(), delta); hrtimer_start(&cpu->hrt, expires, HRTIMER_MODE_ABS); } /* This is the function called when the Guest's timer expires. */ static enum hrtimer_restart clockdev_fn(struct hrtimer *timer) { struct lg_cpu *cpu = container_of(timer, struct lg_cpu, hrt); /* Remember the first interrupt is the timer interrupt. */ set_bit(0, cpu->lg->irqs_pending); /* If the Guest is actually stopped, we need to wake it up. */ if (cpu->lg->halted) wake_up_process(cpu->lg->tsk); return HRTIMER_NORESTART; } /* This sets up the timer for this Guest. */ void init_clockdev(struct lg_cpu *cpu) { hrtimer_init(&cpu->hrt, CLOCK_REALTIME, HRTIMER_MODE_ABS); cpu->hrt.function = clockdev_fn; }