/*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 <linux/uaccess.h> #include <linux/interrupt.h> #include <linux/module.h> #include <linux/sched.h> #include "lg.h" /* Allow Guests to use a non-128 (ie. non-Linux) syscall trap. */ static unsigned int syscall_vector = IA32_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 bool 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 lg_cpu *cpu, unsigned long *gstack, u32 val) { /* Stack grows upwards: move stack then write value. */ *gstack -= 4; lgwrite(cpu, *gstack, u32, val); } /*H:210 * The push_guest_interrupt_stack() routine saves Guest state on the stack for * an 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. * * 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 push_guest_interrupt_stack(struct lg_cpu *cpu, bool 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 ((cpu->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 = cpu->esp1; ss = cpu->ss1; origstack = gstack = guest_pa(cpu, 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(cpu, &gstack, cpu->regs->ss); push_guest_stack(cpu, &gstack, cpu->regs->esp); } else { /* We're staying on the same Guest (kernel) stack. */ virtstack = cpu->regs->esp; ss = cpu->regs->ss; origstack = gstack = guest_pa(cpu, 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 = cpu->regs->eflags; if (get_user(irq_enable, &cpu->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(cpu, &gstack, eflags); push_guest_stack(cpu, &gstack, cpu->regs->cs); push_guest_stack(cpu, &gstack, cpu->regs->eip); /* For the six traps which supply an error code, we push that, too. */ if (has_err) push_guest_stack(cpu, &gstack, cpu->regs->errcode); /* Adjust the stack pointer and stack segment. */ cpu->regs->ss = ss; cpu->regs->esp = virtstack + (gstack - origstack); } /* * This actually makes the Guest start executing the given interrupt/trap * handler. * * "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. */ static void guest_run_interrupt(struct lg_cpu *cpu, u32 lo, u32 hi) { /* If we're already in the kernel, we don't change stacks. */ if ((cpu->regs->ss&0x3) != GUEST_PL) cpu->regs->ss = cpu->esp1; /* * Set the code segment and the address to execute. */ cpu->regs->cs = (__KERNEL_CS|GUEST_PL); cpu->regs->eip = idt_address(lo, hi); /* * Trapping always clears these flags: * TF: Trap flag * VM: Virtual 8086 mode * RF: Resume * NT: Nested task. */ cpu->regs->eflags &= ~(X86_EFLAGS_TF|X86_EFLAGS_VM|X86_EFLAGS_RF|X86_EFLAGS_NT); /* * 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, &cpu->lg->lguest_data->irq_enabled)) kill_guest(cpu, "Disabling interrupts"); } /* This restores the eflags word which was pushed on the stack by a trap */ static void restore_eflags(struct lg_cpu *cpu) { /* This is the physical address of the stack. */ unsigned long stack_pa = guest_pa(cpu, cpu->regs->esp); /* * Stack looks like this: * Address Contents * esp EIP * esp + 4 CS * esp + 8 EFLAGS */ cpu->regs->eflags = lgread(cpu, stack_pa + 8, u32); cpu->regs->eflags &= ~(X86_EFLAGS_TF|X86_EFLAGS_VM|X86_EFLAGS_RF|X86_EFLAGS_NT); } /*H:205 * Virtual Interrupts. * * interrupt_pending() returns the first pending interrupt which isn't blocked * by the Guest. It is called before every entry to the Guest, and just before * we go to sleep when the Guest has halted itself. */ unsigned int interrupt_pending(struct lg_cpu *cpu, bool *more) { unsigned int irq; DECLARE_BITMAP(blk, LGUEST_IRQS); /* If the Guest hasn't even initialized yet, we can do nothing. */ if (!cpu->lg->lguest_data) return LGUEST_IRQS; /* * Take our "irqs_pending" array and remove any interrupts the Guest * wants blocked: the result ends up in "blk". */ if (copy_from_user(&blk, cpu->lg->lguest_data->blocked_interrupts, sizeof(blk))) return LGUEST_IRQS; bitmap_andnot(blk, cpu->irqs_pending, blk, LGUEST_IRQS); /* Find the first interrupt. */ irq = find_first_bit(blk, LGUEST_IRQS); *more = find_next_bit(blk, LGUEST_IRQS, irq+1); return irq; } /* * This actually diverts the Guest to running an interrupt handler, once an * interrupt has been identified by interrupt_pending(). */ void try_deliver_interrupt(struct lg_cpu *cpu, unsigned int irq, bool more) { struct desc_struct *idt; BUG_ON(irq >= LGUEST_IRQS); /* If they're halted, interrupts restart them. */ if (cpu->halted) { /* Re-enable interrupts. */ if (put_user(X86_EFLAGS_IF, &cpu->lg->lguest_data->irq_enabled)) kill_guest(cpu, "Re-enabling interrupts"); cpu->halted = 0; } else { /* Otherwise we check if they have interrupts disabled. */ u32 irq_enabled; if (get_user(irq_enabled, &cpu->lg->lguest_data->irq_enabled)) irq_enabled = 0; if (!irq_enabled) { /* Make sure they know an IRQ is pending. */ put_user(X86_EFLAGS_IF, &cpu->lg->lguest_data->irq_pending); 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 = &cpu->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, cpu->irqs_pending); /* * They may be about to iret, where they asked us never to * deliver interrupts. In this case, we can emulate that iret * then immediately deliver the interrupt. This is basically * a noop: the iret would pop the interrupt frame and restore * eflags, and then we'd set it up again. So just restore the * eflags word and jump straight to the handler in this case. * * Denys Vlasenko points out that this isn't quite right: if * the iret was returning to userspace, then that interrupt * would reset the stack pointer (which the Guest told us * about via LHCALL_SET_STACK). But unless the Guest is being * *really* weird, that will be the same as the current stack * anyway. */ if (cpu->regs->eip == cpu->lg->noirq_iret) { restore_eflags(cpu); } else { /* * set_guest_interrupt() takes a flag to say whether * this interrupt pushes an error code onto the stack * as well: virtual interrupts never do. */ push_guest_interrupt_stack(cpu, false); } /* Actually make Guest cpu jump to handler. */ guest_run_interrupt(cpu, idt->a, idt->b); } /* * 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(cpu); /* * If there are no other interrupts we want to deliver, clear * the pending flag. */ if (!more) put_user(0, &cpu->lg->lguest_data->irq_pending); } /* And this is the routine when we want to set an interrupt for the Guest. */ void set_interrupt(struct lg_cpu *cpu, unsigned int irq) { /* * Next time the Guest runs, the core code will see if it can deliver * this interrupt. */ set_bit(irq, cpu->irqs_pending); /* * Make sure it sees it; it might be asleep (eg. halted), or running * the Guest right now, in which case kick_process() will knock it out. */ if (!wake_up_process(cpu->tsk)) kick_process(cpu->tsk); } /*:*/ /* * 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 IA32_SYSCALL_VECTOR or reserved vector? */ return num == IA32_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 != IA32_SYSCALL_VECTOR) { if (test_bit(syscall_vector, used_vectors) || vector_used_by_percpu_irq(syscall_vector)) { printk(KERN_ERR "lg: couldn't reserve syscall %u\n", syscall_vector); return -EBUSY; } set_bit(syscall_vector, used_vectors); } return 0; } void free_interrupts(void) { if (syscall_vector != IA32_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 bool has_err(unsigned int trap) { return (trap == 8 || (trap >= 10 && trap <= 14) || trap == 17); } /* deliver_trap() returns true if it could deliver the trap. */ bool deliver_trap(struct lg_cpu *cpu, 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(cpu->arch.idt)) return false; /* * 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(cpu->arch.idt[num].a, cpu->arch.idt[num].b)) return false; push_guest_interrupt_stack(cpu, has_err(num)); guest_run_interrupt(cpu, cpu->arch.idt[num].a, cpu->arch.idt[num].b); return true; } /*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 bool 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 false; /* * 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. The Host could provide a mechanism to register more * "no-interrupt" regions, and the Guest could point the trap gate at * instructions within that region, 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 lg_cpu *cpu) { unsigned int i; /* * Depending on the CONFIG_4KSTACKS option, the Guest can have one or * two pages of stack space. */ for (i = 0; i < cpu->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(cpu, cpu->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 guest TSS to use that * stack. The TSS 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 lg_cpu *cpu, u32 seg, u32 esp, unsigned int pages) { /* * You're not allowed a stack segment with privilege level 0: bad Guest! */ if ((seg & 0x3) != GUEST_PL) kill_guest(cpu, "bad stack segment %i", seg); /* We only expect one or two stack pages. */ if (pages > 2) kill_guest(cpu, "bad stack pages %u", pages); /* Save where the stack is, and how many pages */ cpu->ss1 = seg; cpu->esp1 = esp; cpu->lg->stack_pages = pages; /* Make sure the new stack pages are mapped */ pin_stack_pages(cpu); } /* * 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 lg_cpu *cpu, 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(cpu, "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 lg_cpu *cpu, 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. */ cpu->changed |= CHANGED_IDT; /* Check that the Guest doesn't try to step outside the bounds. */ if (num >= ARRAY_SIZE(cpu->arch.idt)) kill_guest(cpu, "Setting idt entry %u", num); else set_trap(cpu, &cpu->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, const struct desc_struct *base) { /* 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); else if (base) /* * Copy privilege level from what Guest asked for. This allows * debug (int 3) traps from Guest userspace, for example. */ flags |= (base->b & 0x6000); /* 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], NULL); } /*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 lg_cpu *cpu, 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(cpu->arch.idt); i++) { const struct desc_struct *gidt = &cpu->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 it can't go direct, we still need to copy the priv. level: * they might want to give userspace access to a software * interrupt. */ if (idt_type(gidt->a, gidt->b) == 0xF) idt[i] = *gidt; else default_idt_entry(&idt[i], i, def[i], gidt); } } /*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_interrupt(cpu, 0); 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; }