linux/drivers/lguest/interrupts_and_traps.c
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   1/*P:800
   2 * Interrupts (traps) are complicated enough to earn their own file.
   3 * There are three classes of interrupts:
   4 *
   5 * 1) Real hardware interrupts which occur while we're running the Guest,
   6 * 2) Interrupts for virtual devices attached to the Guest, and
   7 * 3) Traps and faults from the Guest.
   8 *
   9 * Real hardware interrupts must be delivered to the Host, not the Guest.
  10 * Virtual interrupts must be delivered to the Guest, but we make them look
  11 * just like real hardware would deliver them.  Traps from the Guest can be set
  12 * up to go directly back into the Guest, but sometimes the Host wants to see
  13 * them first, so we also have a way of "reflecting" them into the Guest as if
  14 * they had been delivered to it directly.
  15:*/
  16#include <linux/uaccess.h>
  17#include <linux/interrupt.h>
  18#include <linux/module.h>
  19#include <linux/sched.h>
  20#include "lg.h"
  21
  22/* Allow Guests to use a non-128 (ie. non-Linux) syscall trap. */
  23static unsigned int syscall_vector = SYSCALL_VECTOR;
  24module_param(syscall_vector, uint, 0444);
  25
  26/* The address of the interrupt handler is split into two bits: */
  27static unsigned long idt_address(u32 lo, u32 hi)
  28{
  29        return (lo & 0x0000FFFF) | (hi & 0xFFFF0000);
  30}
  31
  32/*
  33 * The "type" of the interrupt handler is a 4 bit field: we only support a
  34 * couple of types.
  35 */
  36static int idt_type(u32 lo, u32 hi)
  37{
  38        return (hi >> 8) & 0xF;
  39}
  40
  41/* An IDT entry can't be used unless the "present" bit is set. */
  42static bool idt_present(u32 lo, u32 hi)
  43{
  44        return (hi & 0x8000);
  45}
  46
  47/*
  48 * We need a helper to "push" a value onto the Guest's stack, since that's a
  49 * big part of what delivering an interrupt does.
  50 */
  51static void push_guest_stack(struct lg_cpu *cpu, unsigned long *gstack, u32 val)
  52{
  53        /* Stack grows upwards: move stack then write value. */
  54        *gstack -= 4;
  55        lgwrite(cpu, *gstack, u32, val);
  56}
  57
  58/*H:210
  59 * The set_guest_interrupt() routine actually delivers the interrupt or
  60 * trap.  The mechanics of delivering traps and interrupts to the Guest are the
  61 * same, except some traps have an "error code" which gets pushed onto the
  62 * stack as well: the caller tells us if this is one.
  63 *
  64 * "lo" and "hi" are the two parts of the Interrupt Descriptor Table for this
  65 * interrupt or trap.  It's split into two parts for traditional reasons: gcc
  66 * on i386 used to be frightened by 64 bit numbers.
  67 *
  68 * We set up the stack just like the CPU does for a real interrupt, so it's
  69 * identical for the Guest (and the standard "iret" instruction will undo
  70 * it).
  71 */
  72static void set_guest_interrupt(struct lg_cpu *cpu, u32 lo, u32 hi,
  73                                bool has_err)
  74{
  75        unsigned long gstack, origstack;
  76        u32 eflags, ss, irq_enable;
  77        unsigned long virtstack;
  78
  79        /*
  80         * There are two cases for interrupts: one where the Guest is already
  81         * in the kernel, and a more complex one where the Guest is in
  82         * userspace.  We check the privilege level to find out.
  83         */
  84        if ((cpu->regs->ss&0x3) != GUEST_PL) {
  85                /*
  86                 * The Guest told us their kernel stack with the SET_STACK
  87                 * hypercall: both the virtual address and the segment.
  88                 */
  89                virtstack = cpu->esp1;
  90                ss = cpu->ss1;
  91
  92                origstack = gstack = guest_pa(cpu, virtstack);
  93                /*
  94                 * We push the old stack segment and pointer onto the new
  95                 * stack: when the Guest does an "iret" back from the interrupt
  96                 * handler the CPU will notice they're dropping privilege
  97                 * levels and expect these here.
  98                 */
  99                push_guest_stack(cpu, &gstack, cpu->regs->ss);
 100                push_guest_stack(cpu, &gstack, cpu->regs->esp);
 101        } else {
 102                /* We're staying on the same Guest (kernel) stack. */
 103                virtstack = cpu->regs->esp;
 104                ss = cpu->regs->ss;
 105
 106                origstack = gstack = guest_pa(cpu, virtstack);
 107        }
 108
 109        /*
 110         * Remember that we never let the Guest actually disable interrupts, so
 111         * the "Interrupt Flag" bit is always set.  We copy that bit from the
 112         * Guest's "irq_enabled" field into the eflags word: we saw the Guest
 113         * copy it back in "lguest_iret".
 114         */
 115        eflags = cpu->regs->eflags;
 116        if (get_user(irq_enable, &cpu->lg->lguest_data->irq_enabled) == 0
 117            && !(irq_enable & X86_EFLAGS_IF))
 118                eflags &= ~X86_EFLAGS_IF;
 119
 120        /*
 121         * An interrupt is expected to push three things on the stack: the old
 122         * "eflags" word, the old code segment, and the old instruction
 123         * pointer.
 124         */
 125        push_guest_stack(cpu, &gstack, eflags);
 126        push_guest_stack(cpu, &gstack, cpu->regs->cs);
 127        push_guest_stack(cpu, &gstack, cpu->regs->eip);
 128
 129        /* For the six traps which supply an error code, we push that, too. */
 130        if (has_err)
 131                push_guest_stack(cpu, &gstack, cpu->regs->errcode);
 132
 133        /*
 134         * Now we've pushed all the old state, we change the stack, the code
 135         * segment and the address to execute.
 136         */
 137        cpu->regs->ss = ss;
 138        cpu->regs->esp = virtstack + (gstack - origstack);
 139        cpu->regs->cs = (__KERNEL_CS|GUEST_PL);
 140        cpu->regs->eip = idt_address(lo, hi);
 141
 142        /*
 143         * There are two kinds of interrupt handlers: 0xE is an "interrupt
 144         * gate" which expects interrupts to be disabled on entry.
 145         */
 146        if (idt_type(lo, hi) == 0xE)
 147                if (put_user(0, &cpu->lg->lguest_data->irq_enabled))
 148                        kill_guest(cpu, "Disabling interrupts");
 149}
 150
 151/*H:205
 152 * Virtual Interrupts.
 153 *
 154 * interrupt_pending() returns the first pending interrupt which isn't blocked
 155 * by the Guest.  It is called before every entry to the Guest, and just before
 156 * we go to sleep when the Guest has halted itself.
 157 */
 158unsigned int interrupt_pending(struct lg_cpu *cpu, bool *more)
 159{
 160        unsigned int irq;
 161        DECLARE_BITMAP(blk, LGUEST_IRQS);
 162
 163        /* If the Guest hasn't even initialized yet, we can do nothing. */
 164        if (!cpu->lg->lguest_data)
 165                return LGUEST_IRQS;
 166
 167        /*
 168         * Take our "irqs_pending" array and remove any interrupts the Guest
 169         * wants blocked: the result ends up in "blk".
 170         */
 171        if (copy_from_user(&blk, cpu->lg->lguest_data->blocked_interrupts,
 172                           sizeof(blk)))
 173                return LGUEST_IRQS;
 174        bitmap_andnot(blk, cpu->irqs_pending, blk, LGUEST_IRQS);
 175
 176        /* Find the first interrupt. */
 177        irq = find_first_bit(blk, LGUEST_IRQS);
 178        *more = find_next_bit(blk, LGUEST_IRQS, irq+1);
 179
 180        return irq;
 181}
 182
 183/*
 184 * This actually diverts the Guest to running an interrupt handler, once an
 185 * interrupt has been identified by interrupt_pending().
 186 */
 187void try_deliver_interrupt(struct lg_cpu *cpu, unsigned int irq, bool more)
 188{
 189        struct desc_struct *idt;
 190
 191        BUG_ON(irq >= LGUEST_IRQS);
 192
 193        /*
 194         * They may be in the middle of an iret, where they asked us never to
 195         * deliver interrupts.
 196         */
 197        if (cpu->regs->eip >= cpu->lg->noirq_start &&
 198           (cpu->regs->eip < cpu->lg->noirq_end))
 199                return;
 200
 201        /* If they're halted, interrupts restart them. */
 202        if (cpu->halted) {
 203                /* Re-enable interrupts. */
 204                if (put_user(X86_EFLAGS_IF, &cpu->lg->lguest_data->irq_enabled))
 205                        kill_guest(cpu, "Re-enabling interrupts");
 206                cpu->halted = 0;
 207        } else {
 208                /* Otherwise we check if they have interrupts disabled. */
 209                u32 irq_enabled;
 210                if (get_user(irq_enabled, &cpu->lg->lguest_data->irq_enabled))
 211                        irq_enabled = 0;
 212                if (!irq_enabled) {
 213                        /* Make sure they know an IRQ is pending. */
 214                        put_user(X86_EFLAGS_IF,
 215                                 &cpu->lg->lguest_data->irq_pending);
 216                        return;
 217                }
 218        }
 219
 220        /*
 221         * Look at the IDT entry the Guest gave us for this interrupt.  The
 222         * first 32 (FIRST_EXTERNAL_VECTOR) entries are for traps, so we skip
 223         * over them.
 224         */
 225        idt = &cpu->arch.idt[FIRST_EXTERNAL_VECTOR+irq];
 226        /* If they don't have a handler (yet?), we just ignore it */
 227        if (idt_present(idt->a, idt->b)) {
 228                /* OK, mark it no longer pending and deliver it. */
 229                clear_bit(irq, cpu->irqs_pending);
 230                /*
 231                 * set_guest_interrupt() takes the interrupt descriptor and a
 232                 * flag to say whether this interrupt pushes an error code onto
 233                 * the stack as well: virtual interrupts never do.
 234                 */
 235                set_guest_interrupt(cpu, idt->a, idt->b, false);
 236        }
 237
 238        /*
 239         * Every time we deliver an interrupt, we update the timestamp in the
 240         * Guest's lguest_data struct.  It would be better for the Guest if we
 241         * did this more often, but it can actually be quite slow: doing it
 242         * here is a compromise which means at least it gets updated every
 243         * timer interrupt.
 244         */
 245        write_timestamp(cpu);
 246
 247        /*
 248         * If there are no other interrupts we want to deliver, clear
 249         * the pending flag.
 250         */
 251        if (!more)
 252                put_user(0, &cpu->lg->lguest_data->irq_pending);
 253}
 254
 255/* And this is the routine when we want to set an interrupt for the Guest. */
 256void set_interrupt(struct lg_cpu *cpu, unsigned int irq)
 257{
 258        /*
 259         * Next time the Guest runs, the core code will see if it can deliver
 260         * this interrupt.
 261         */
 262        set_bit(irq, cpu->irqs_pending);
 263
 264        /*
 265         * Make sure it sees it; it might be asleep (eg. halted), or running
 266         * the Guest right now, in which case kick_process() will knock it out.
 267         */
 268        if (!wake_up_process(cpu->tsk))
 269                kick_process(cpu->tsk);
 270}
 271/*:*/
 272
 273/*
 274 * Linux uses trap 128 for system calls.  Plan9 uses 64, and Ron Minnich sent
 275 * me a patch, so we support that too.  It'd be a big step for lguest if half
 276 * the Plan 9 user base were to start using it.
 277 *
 278 * Actually now I think of it, it's possible that Ron *is* half the Plan 9
 279 * userbase.  Oh well.
 280 */
 281static bool could_be_syscall(unsigned int num)
 282{
 283        /* Normal Linux SYSCALL_VECTOR or reserved vector? */
 284        return num == SYSCALL_VECTOR || num == syscall_vector;
 285}
 286
 287/* The syscall vector it wants must be unused by Host. */
 288bool check_syscall_vector(struct lguest *lg)
 289{
 290        u32 vector;
 291
 292        if (get_user(vector, &lg->lguest_data->syscall_vec))
 293                return false;
 294
 295        return could_be_syscall(vector);
 296}
 297
 298int init_interrupts(void)
 299{
 300        /* If they want some strange system call vector, reserve it now */
 301        if (syscall_vector != SYSCALL_VECTOR) {
 302                if (test_bit(syscall_vector, used_vectors) ||
 303                    vector_used_by_percpu_irq(syscall_vector)) {
 304                        printk(KERN_ERR "lg: couldn't reserve syscall %u\n",
 305                                 syscall_vector);
 306                        return -EBUSY;
 307                }
 308                set_bit(syscall_vector, used_vectors);
 309        }
 310
 311        return 0;
 312}
 313
 314void free_interrupts(void)
 315{
 316        if (syscall_vector != SYSCALL_VECTOR)
 317                clear_bit(syscall_vector, used_vectors);
 318}
 319
 320/*H:220
 321 * Now we've got the routines to deliver interrupts, delivering traps like
 322 * page fault is easy.  The only trick is that Intel decided that some traps
 323 * should have error codes:
 324 */
 325static bool has_err(unsigned int trap)
 326{
 327        return (trap == 8 || (trap >= 10 && trap <= 14) || trap == 17);
 328}
 329
 330/* deliver_trap() returns true if it could deliver the trap. */
 331bool deliver_trap(struct lg_cpu *cpu, unsigned int num)
 332{
 333        /*
 334         * Trap numbers are always 8 bit, but we set an impossible trap number
 335         * for traps inside the Switcher, so check that here.
 336         */
 337        if (num >= ARRAY_SIZE(cpu->arch.idt))
 338                return false;
 339
 340        /*
 341         * Early on the Guest hasn't set the IDT entries (or maybe it put a
 342         * bogus one in): if we fail here, the Guest will be killed.
 343         */
 344        if (!idt_present(cpu->arch.idt[num].a, cpu->arch.idt[num].b))
 345                return false;
 346        set_guest_interrupt(cpu, cpu->arch.idt[num].a,
 347                            cpu->arch.idt[num].b, has_err(num));
 348        return true;
 349}
 350
 351/*H:250
 352 * Here's the hard part: returning to the Host every time a trap happens
 353 * and then calling deliver_trap() and re-entering the Guest is slow.
 354 * Particularly because Guest userspace system calls are traps (usually trap
 355 * 128).
 356 *
 357 * So we'd like to set up the IDT to tell the CPU to deliver traps directly
 358 * into the Guest.  This is possible, but the complexities cause the size of
 359 * this file to double!  However, 150 lines of code is worth writing for taking
 360 * system calls down from 1750ns to 270ns.  Plus, if lguest didn't do it, all
 361 * the other hypervisors would beat it up at lunchtime.
 362 *
 363 * This routine indicates if a particular trap number could be delivered
 364 * directly.
 365 */
 366static bool direct_trap(unsigned int num)
 367{
 368        /*
 369         * Hardware interrupts don't go to the Guest at all (except system
 370         * call).
 371         */
 372        if (num >= FIRST_EXTERNAL_VECTOR && !could_be_syscall(num))
 373                return false;
 374
 375        /*
 376         * The Host needs to see page faults (for shadow paging and to save the
 377         * fault address), general protection faults (in/out emulation) and
 378         * device not available (TS handling) and of course, the hypercall trap.
 379         */
 380        return num != 14 && num != 13 && num != 7 && num != LGUEST_TRAP_ENTRY;
 381}
 382/*:*/
 383
 384/*M:005
 385 * The Guest has the ability to turn its interrupt gates into trap gates,
 386 * if it is careful.  The Host will let trap gates can go directly to the
 387 * Guest, but the Guest needs the interrupts atomically disabled for an
 388 * interrupt gate.  It can do this by pointing the trap gate at instructions
 389 * within noirq_start and noirq_end, where it can safely disable interrupts.
 390 */
 391
 392/*M:006
 393 * The Guests do not use the sysenter (fast system call) instruction,
 394 * because it's hardcoded to enter privilege level 0 and so can't go direct.
 395 * It's about twice as fast as the older "int 0x80" system call, so it might
 396 * still be worthwhile to handle it in the Switcher and lcall down to the
 397 * Guest.  The sysenter semantics are hairy tho: search for that keyword in
 398 * entry.S
 399:*/
 400
 401/*H:260
 402 * When we make traps go directly into the Guest, we need to make sure
 403 * the kernel stack is valid (ie. mapped in the page tables).  Otherwise, the
 404 * CPU trying to deliver the trap will fault while trying to push the interrupt
 405 * words on the stack: this is called a double fault, and it forces us to kill
 406 * the Guest.
 407 *
 408 * Which is deeply unfair, because (literally!) it wasn't the Guests' fault.
 409 */
 410void pin_stack_pages(struct lg_cpu *cpu)
 411{
 412        unsigned int i;
 413
 414        /*
 415         * Depending on the CONFIG_4KSTACKS option, the Guest can have one or
 416         * two pages of stack space.
 417         */
 418        for (i = 0; i < cpu->lg->stack_pages; i++)
 419                /*
 420                 * The stack grows *upwards*, so the address we're given is the
 421                 * start of the page after the kernel stack.  Subtract one to
 422                 * get back onto the first stack page, and keep subtracting to
 423                 * get to the rest of the stack pages.
 424                 */
 425                pin_page(cpu, cpu->esp1 - 1 - i * PAGE_SIZE);
 426}
 427
 428/*
 429 * Direct traps also mean that we need to know whenever the Guest wants to use
 430 * a different kernel stack, so we can change the guest TSS to use that
 431 * stack.  The TSS entries expect a virtual address, so unlike most addresses
 432 * the Guest gives us, the "esp" (stack pointer) value here is virtual, not
 433 * physical.
 434 *
 435 * In Linux each process has its own kernel stack, so this happens a lot: we
 436 * change stacks on each context switch.
 437 */
 438void guest_set_stack(struct lg_cpu *cpu, u32 seg, u32 esp, unsigned int pages)
 439{
 440        /*
 441         * You're not allowed a stack segment with privilege level 0: bad Guest!
 442         */
 443        if ((seg & 0x3) != GUEST_PL)
 444                kill_guest(cpu, "bad stack segment %i", seg);
 445        /* We only expect one or two stack pages. */
 446        if (pages > 2)
 447                kill_guest(cpu, "bad stack pages %u", pages);
 448        /* Save where the stack is, and how many pages */
 449        cpu->ss1 = seg;
 450        cpu->esp1 = esp;
 451        cpu->lg->stack_pages = pages;
 452        /* Make sure the new stack pages are mapped */
 453        pin_stack_pages(cpu);
 454}
 455
 456/*
 457 * All this reference to mapping stacks leads us neatly into the other complex
 458 * part of the Host: page table handling.
 459 */
 460
 461/*H:235
 462 * This is the routine which actually checks the Guest's IDT entry and
 463 * transfers it into the entry in "struct lguest":
 464 */
 465static void set_trap(struct lg_cpu *cpu, struct desc_struct *trap,
 466                     unsigned int num, u32 lo, u32 hi)
 467{
 468        u8 type = idt_type(lo, hi);
 469
 470        /* We zero-out a not-present entry */
 471        if (!idt_present(lo, hi)) {
 472                trap->a = trap->b = 0;
 473                return;
 474        }
 475
 476        /* We only support interrupt and trap gates. */
 477        if (type != 0xE && type != 0xF)
 478                kill_guest(cpu, "bad IDT type %i", type);
 479
 480        /*
 481         * We only copy the handler address, present bit, privilege level and
 482         * type.  The privilege level controls where the trap can be triggered
 483         * manually with an "int" instruction.  This is usually GUEST_PL,
 484         * except for system calls which userspace can use.
 485         */
 486        trap->a = ((__KERNEL_CS|GUEST_PL)<<16) | (lo&0x0000FFFF);
 487        trap->b = (hi&0xFFFFEF00);
 488}
 489
 490/*H:230
 491 * While we're here, dealing with delivering traps and interrupts to the
 492 * Guest, we might as well complete the picture: how the Guest tells us where
 493 * it wants them to go.  This would be simple, except making traps fast
 494 * requires some tricks.
 495 *
 496 * We saw the Guest setting Interrupt Descriptor Table (IDT) entries with the
 497 * LHCALL_LOAD_IDT_ENTRY hypercall before: that comes here.
 498 */
 499void load_guest_idt_entry(struct lg_cpu *cpu, unsigned int num, u32 lo, u32 hi)
 500{
 501        /*
 502         * Guest never handles: NMI, doublefault, spurious interrupt or
 503         * hypercall.  We ignore when it tries to set them.
 504         */
 505        if (num == 2 || num == 8 || num == 15 || num == LGUEST_TRAP_ENTRY)
 506                return;
 507
 508        /*
 509         * Mark the IDT as changed: next time the Guest runs we'll know we have
 510         * to copy this again.
 511         */
 512        cpu->changed |= CHANGED_IDT;
 513
 514        /* Check that the Guest doesn't try to step outside the bounds. */
 515        if (num >= ARRAY_SIZE(cpu->arch.idt))
 516                kill_guest(cpu, "Setting idt entry %u", num);
 517        else
 518                set_trap(cpu, &cpu->arch.idt[num], num, lo, hi);
 519}
 520
 521/*
 522 * The default entry for each interrupt points into the Switcher routines which
 523 * simply return to the Host.  The run_guest() loop will then call
 524 * deliver_trap() to bounce it back into the Guest.
 525 */
 526static void default_idt_entry(struct desc_struct *idt,
 527                              int trap,
 528                              const unsigned long handler,
 529                              const struct desc_struct *base)
 530{
 531        /* A present interrupt gate. */
 532        u32 flags = 0x8e00;
 533
 534        /*
 535         * Set the privilege level on the entry for the hypercall: this allows
 536         * the Guest to use the "int" instruction to trigger it.
 537         */
 538        if (trap == LGUEST_TRAP_ENTRY)
 539                flags |= (GUEST_PL << 13);
 540        else if (base)
 541                /*
 542                 * Copy privilege level from what Guest asked for.  This allows
 543                 * debug (int 3) traps from Guest userspace, for example.
 544                 */
 545                flags |= (base->b & 0x6000);
 546
 547        /* Now pack it into the IDT entry in its weird format. */
 548        idt->a = (LGUEST_CS<<16) | (handler&0x0000FFFF);
 549        idt->b = (handler&0xFFFF0000) | flags;
 550}
 551
 552/* When the Guest first starts, we put default entries into the IDT. */
 553void setup_default_idt_entries(struct lguest_ro_state *state,
 554                               const unsigned long *def)
 555{
 556        unsigned int i;
 557
 558        for (i = 0; i < ARRAY_SIZE(state->guest_idt); i++)
 559                default_idt_entry(&state->guest_idt[i], i, def[i], NULL);
 560}
 561
 562/*H:240
 563 * We don't use the IDT entries in the "struct lguest" directly, instead
 564 * we copy them into the IDT which we've set up for Guests on this CPU, just
 565 * before we run the Guest.  This routine does that copy.
 566 */
 567void copy_traps(const struct lg_cpu *cpu, struct desc_struct *idt,
 568                const unsigned long *def)
 569{
 570        unsigned int i;
 571
 572        /*
 573         * We can simply copy the direct traps, otherwise we use the default
 574         * ones in the Switcher: they will return to the Host.
 575         */
 576        for (i = 0; i < ARRAY_SIZE(cpu->arch.idt); i++) {
 577                const struct desc_struct *gidt = &cpu->arch.idt[i];
 578
 579                /* If no Guest can ever override this trap, leave it alone. */
 580                if (!direct_trap(i))
 581                        continue;
 582
 583                /*
 584                 * Only trap gates (type 15) can go direct to the Guest.
 585                 * Interrupt gates (type 14) disable interrupts as they are
 586                 * entered, which we never let the Guest do.  Not present
 587                 * entries (type 0x0) also can't go direct, of course.
 588                 *
 589                 * If it can't go direct, we still need to copy the priv. level:
 590                 * they might want to give userspace access to a software
 591                 * interrupt.
 592                 */
 593                if (idt_type(gidt->a, gidt->b) == 0xF)
 594                        idt[i] = *gidt;
 595                else
 596                        default_idt_entry(&idt[i], i, def[i], gidt);
 597        }
 598}
 599
 600/*H:200
 601 * The Guest Clock.
 602 *
 603 * There are two sources of virtual interrupts.  We saw one in lguest_user.c:
 604 * the Launcher sending interrupts for virtual devices.  The other is the Guest
 605 * timer interrupt.
 606 *
 607 * The Guest uses the LHCALL_SET_CLOCKEVENT hypercall to tell us how long to
 608 * the next timer interrupt (in nanoseconds).  We use the high-resolution timer
 609 * infrastructure to set a callback at that time.
 610 *
 611 * 0 means "turn off the clock".
 612 */
 613void guest_set_clockevent(struct lg_cpu *cpu, unsigned long delta)
 614{
 615        ktime_t expires;
 616
 617        if (unlikely(delta == 0)) {
 618                /* Clock event device is shutting down. */
 619                hrtimer_cancel(&cpu->hrt);
 620                return;
 621        }
 622
 623        /*
 624         * We use wallclock time here, so the Guest might not be running for
 625         * all the time between now and the timer interrupt it asked for.  This
 626         * is almost always the right thing to do.
 627         */
 628        expires = ktime_add_ns(ktime_get_real(), delta);
 629        hrtimer_start(&cpu->hrt, expires, HRTIMER_MODE_ABS);
 630}
 631
 632/* This is the function called when the Guest's timer expires. */
 633static enum hrtimer_restart clockdev_fn(struct hrtimer *timer)
 634{
 635        struct lg_cpu *cpu = container_of(timer, struct lg_cpu, hrt);
 636
 637        /* Remember the first interrupt is the timer interrupt. */
 638        set_interrupt(cpu, 0);
 639        return HRTIMER_NORESTART;
 640}
 641
 642/* This sets up the timer for this Guest. */
 643void init_clockdev(struct lg_cpu *cpu)
 644{
 645        hrtimer_init(&cpu->hrt, CLOCK_REALTIME, HRTIMER_MODE_ABS);
 646        cpu->hrt.function = clockdev_fn;
 647}
 648
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