diff options
Diffstat (limited to 'drivers/lguest/interrupts_and_traps.c')
| -rw-r--r-- | drivers/lguest/interrupts_and_traps.c | 702 |
1 files changed, 702 insertions, 0 deletions
diff --git a/drivers/lguest/interrupts_and_traps.c b/drivers/lguest/interrupts_and_traps.c new file mode 100644 index 000000000..5e7559be2 --- /dev/null +++ b/drivers/lguest/interrupts_and_traps.c @@ -0,0 +1,702 @@ +/*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 = 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 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) { + 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 != 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; +} |