Merge remote-tracking branch 'asoc/topic/topology' into asoc-next

[mirror_ubuntu-zesty-kernel.git] / 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 <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.
 */
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.
 *
 * Unfortunately, Linux 4.6 started using an interrupt gate instead of a
 * trap gate for syscalls, so this trick is ineffective.  See Mastery for
 * how we could do this anyway...
 */
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;
}