lguest: documentation VII: FIXMEs

[safe/jmp/linux-2.6] / drivers / lguest / interrupts_and_traps.c

/*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 "lg.h"

/* 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_u32(lg, *gstack, 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;
        u32 eflags, ss, irq_enable;

        /* 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 */
                gstack = guest_pa(lg, lg->esp1);
                ss = lg->ss1;
                /* 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. */
                gstack = guest_pa(lg, lg->regs->esp);
                ss = lg->regs->ss;
        }

        /* 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: the Guest copies
         * 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 = gstack + lg->page_offset;
        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:200
 * 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->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);
        }
}

/*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)
{
        u32 lo = lg->idt[num].a, hi = lg->idt[num].b;

        /* 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(lo, hi))
                return 0;
        set_guest_interrupt(lg, lo, hi, 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 (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 tease it.
 *
 * This routine determines if a trap can be delivered directly. */
static int direct_trap(const struct lguest *lg,
                       const struct desc_struct *trap,
                       unsigned int num)
{
        /* Hardware interrupts don't go to the Guest at all (except system
         * call). */
        if (num >= FIRST_EXTERNAL_VECTOR && num != SYSCALL_VECTOR)
                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. */
        if (num == 14 || num == 13 || num == 7 || num == LGUEST_TRAP_ENTRY)
                return 0;

        /* 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 8) */
        return idt_type(trap->a, trap->b) == 0xF;
}
/*:*/

/*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*, hence the subtraction */
                pin_page(lg, lg->esp1 - 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 allowd 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 our 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;

        /* The IDT which we keep in "struct lguest" only contains 32 entries
         * for the traps and LGUEST_IRQS (32) entries for interrupts.  We
         * ignore attempts to set handlers for higher interrupt numbers, except
         * for the system call "interrupt" at 128: we have a special IDT entry
         * for that. */
        if (num < ARRAY_SIZE(lg->idt))
                set_trap(lg, &lg->idt[num], num, lo, hi);
        else if (num == SYSCALL_VECTOR)
                set_trap(lg, &lg->syscall_idt, 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 < FIRST_EXTERNAL_VECTOR; i++) {
                if (direct_trap(lg, &lg->idt[i], i))
                        idt[i] = lg->idt[i];
                else
                        default_idt_entry(&idt[i], i, def[i]);
        }

        /* Don't forget the system call trap!  The IDT entries for other
         * interupts never change, so no need to copy them. */
        i = SYSCALL_VECTOR;
        if (direct_trap(lg, &lg->syscall_idt, i))
                idt[i] = lg->syscall_idt;
        else
                default_idt_entry(&idt[i], i, def[i]);
}

void guest_set_clockevent(struct lguest *lg, unsigned long delta)
{
        ktime_t expires;

        if (unlikely(delta == 0)) {
                /* Clock event device is shutting down. */
                hrtimer_cancel(&lg->hrt);
                return;
        }

        expires = ktime_add_ns(ktime_get_real(), delta);
        hrtimer_start(&lg->hrt, expires, HRTIMER_MODE_ABS);
}

static enum hrtimer_restart clockdev_fn(struct hrtimer *timer)
{
        struct lguest *lg = container_of(timer, struct lguest, hrt);

        set_bit(0, lg->irqs_pending);
        if (lg->halted)
                wake_up_process(lg->tsk);
        return HRTIMER_NORESTART;
}

void init_clockdev(struct lguest *lg)
{
        hrtimer_init(&lg->hrt, CLOCK_REALTIME, HRTIMER_MODE_ABS);
        lg->hrt.function = clockdev_fn;
}