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/*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 "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 lguest *lg, unsigned long delta)

{

        ktime_t expires;

        if (unlikely(delta == 0)) {

                /* Clock event device is shutting down. */

                hrtimer_cancel(&lg->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(&lg->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 lguest *lg = container_of(timer, struct lguest, hrt);

        /* Remember the first interrupt is the timer interrupt. */

        set_bit(0, lg->irqs_pending);

        /* If the Guest is actually stopped, we need to wake it up. */

        if (lg->halted)

                wake_up_process(lg->tsk);

        return HRTIMER_NORESTART;

}

/* This sets up the timer for this Guest. */

void init_clockdev(struct lguest *lg)

{

        hrtimer_init(&lg->hrt, CLOCK_REALTIME, HRTIMER_MODE_ABS);

        lg->hrt.function = clockdev_fn;

}