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         * Trapping always clears these flags:
 144         * TF: Trap flag
 145         * VM: Virtual 8086 mode
 146         * RF: Resume
 147         * NT: Nested task.
 148         */
 149        cpu->regs->eflags &=
 150                ~(X86_EFLAGS_TF|X86_EFLAGS_VM|X86_EFLAGS_RF|X86_EFLAGS_NT);
 151
 152        /*
 153         * There are two kinds of interrupt handlers: 0xE is an "interrupt
 154         * gate" which expects interrupts to be disabled on entry.
 155         */
 156        if (idt_type(lo, hi) == 0xE)
 157                if (put_user(0, &cpu->lg->lguest_data->irq_enabled))
 158                        kill_guest(cpu, "Disabling interrupts");
 159}
 160
 161/*H:205
 162 * Virtual Interrupts.
 163 *
 164 * interrupt_pending() returns the first pending interrupt which isn't blocked
 165 * by the Guest.  It is called before every entry to the Guest, and just before
 166 * we go to sleep when the Guest has halted itself.
 167 */
 168unsigned int interrupt_pending(struct lg_cpu *cpu, bool *more)
 169{
 170        unsigned int irq;
 171        DECLARE_BITMAP(blk, LGUEST_IRQS);
 172
 173        /* If the Guest hasn't even initialized yet, we can do nothing. */
 174        if (!cpu->lg->lguest_data)
 175                return LGUEST_IRQS;
 176
 177        /*
 178         * Take our "irqs_pending" array and remove any interrupts the Guest
 179         * wants blocked: the result ends up in "blk".
 180         */
 181        if (copy_from_user(&blk, cpu->lg->lguest_data->blocked_interrupts,
 182                           sizeof(blk)))
 183                return LGUEST_IRQS;
 184        bitmap_andnot(blk, cpu->irqs_pending, blk, LGUEST_IRQS);
 185
 186        /* Find the first interrupt. */
 187        irq = find_first_bit(blk, LGUEST_IRQS);
 188        *more = find_next_bit(blk, LGUEST_IRQS, irq+1);
 189
 190        return irq;
 191}
 192
 193/*
 194 * This actually diverts the Guest to running an interrupt handler, once an
 195 * interrupt has been identified by interrupt_pending().
 196 */
 197void try_deliver_interrupt(struct lg_cpu *cpu, unsigned int irq, bool more)
 198{
 199        struct desc_struct *idt;
 200
 201        BUG_ON(irq >= LGUEST_IRQS);
 202
 203        /*
 204         * They may be in the middle of an iret, where they asked us never to
 205         * deliver interrupts.
 206         */
 207        if (cpu->regs->eip >= cpu->lg->noirq_start &&
 208           (cpu->regs->eip < cpu->lg->noirq_end))
 209                return;
 210
 211        /* If they're halted, interrupts restart them. */
 212        if (cpu->halted) {
 213                /* Re-enable interrupts. */
 214                if (put_user(X86_EFLAGS_IF, &cpu->lg->lguest_data->irq_enabled))
 215                        kill_guest(cpu, "Re-enabling interrupts");
 216                cpu->halted = 0;
 217        } else {
 218                /* Otherwise we check if they have interrupts disabled. */
 219                u32 irq_enabled;
 220                if (get_user(irq_enabled, &cpu->lg->lguest_data->irq_enabled))
 221                        irq_enabled = 0;
 222                if (!irq_enabled) {
 223                        /* Make sure they know an IRQ is pending. */
 224                        put_user(X86_EFLAGS_IF,
 225                                 &cpu->lg->lguest_data->irq_pending);
 226                        return;
 227                }
 228        }
 229
 230        /*
 231         * Look at the IDT entry the Guest gave us for this interrupt.  The
 232         * first 32 (FIRST_EXTERNAL_VECTOR) entries are for traps, so we skip
 233         * over them.
 234         */
 235        idt = &cpu->arch.idt[FIRST_EXTERNAL_VECTOR+irq];
 236        /* If they don't have a handler (yet?), we just ignore it */
 237        if (idt_present(idt->a, idt->b)) {
 238                /* OK, mark it no longer pending and deliver it. */
 239                clear_bit(irq, cpu->irqs_pending);
 240                /*
 241                 * set_guest_interrupt() takes the interrupt descriptor and a
 242                 * flag to say whether this interrupt pushes an error code onto
 243                 * the stack as well: virtual interrupts never do.
 244                 */
 245                set_guest_interrupt(cpu, idt->a, idt->b, false);
 246        }
 247
 248        /*
 249         * Every time we deliver an interrupt, we update the timestamp in the
 250         * Guest's lguest_data struct.  It would be better for the Guest if we
 251         * did this more often, but it can actually be quite slow: doing it
 252         * here is a compromise which means at least it gets updated every
 253         * timer interrupt.
 254         */
 255        write_timestamp(cpu);
 256
 257        /*
 258         * If there are no other interrupts we want to deliver, clear
 259         * the pending flag.
 260         */
 261        if (!more)
 262                put_user(0, &cpu->lg->lguest_data->irq_pending);
 263}
 264
 265/* And this is the routine when we want to set an interrupt for the Guest. */
 266void set_interrupt(struct lg_cpu *cpu, unsigned int irq)
 267{
 268        /*
 269         * Next time the Guest runs, the core code will see if it can deliver
 270         * this interrupt.
 271         */
 272        set_bit(irq, cpu->irqs_pending);
 273
 274        /*
 275         * Make sure it sees it; it might be asleep (eg. halted), or running
 276         * the Guest right now, in which case kick_process() will knock it out.
 277         */
 278        if (!wake_up_process(cpu->tsk))
 279                kick_process(cpu->tsk);
 280}
 281/*:*/
 282
 283/*
 284 * Linux uses trap 128 for system calls.  Plan9 uses 64, and Ron Minnich sent
 285 * me a patch, so we support that too.  It'd be a big step for lguest if half
 286 * the Plan 9 user base were to start using it.
 287 *
 288 * Actually now I think of it, it's possible that Ron *is* half the Plan 9
 289 * userbase.  Oh well.
 290 */
 291static bool could_be_syscall(unsigned int num)
 292{
 293        /* Normal Linux SYSCALL_VECTOR or reserved vector? */
 294        return num == SYSCALL_VECTOR || num == syscall_vector;
 295}
 296
 297/* The syscall vector it wants must be unused by Host. */
 298bool check_syscall_vector(struct lguest *lg)
 299{
 300        u32 vector;
 301
 302        if (get_user(vector, &lg->lguest_data->syscall_vec))
 303                return false;
 304
 305        return could_be_syscall(vector);
 306}
 307
 308int init_interrupts(void)
 309{
 310        /* If they want some strange system call vector, reserve it now */
 311        if (syscall_vector != SYSCALL_VECTOR) {
 312                if (test_bit(syscall_vector, used_vectors) ||
 313                    vector_used_by_percpu_irq(syscall_vector)) {
 314                        printk(KERN_ERR "lg: couldn't reserve syscall %u\n",
 315                                 syscall_vector);
 316                        return -EBUSY;
 317                }
 318                set_bit(syscall_vector, used_vectors);
 319        }
 320
 321        return 0;
 322}
 323
 324void free_interrupts(void)
 325{
 326        if (syscall_vector != SYSCALL_VECTOR)
 327                clear_bit(syscall_vector, used_vectors);
 328}
 329
 330/*H:220
 331 * Now we've got the routines to deliver interrupts, delivering traps like
 332 * page fault is easy.  The only trick is that Intel decided that some traps
 333 * should have error codes:
 334 */
 335static bool has_err(unsigned int trap)
 336{
 337        return (trap == 8 || (trap >= 10 && trap <= 14) || trap == 17);
 338}
 339
 340/* deliver_trap() returns true if it could deliver the trap. */
 341bool deliver_trap(struct lg_cpu *cpu, unsigned int num)
 342{
 343        /*
 344         * Trap numbers are always 8 bit, but we set an impossible trap number
 345         * for traps inside the Switcher, so check that here.
 346         */
 347        if (num >= ARRAY_SIZE(cpu->arch.idt))
 348                return false;
 349
 350        /*
 351         * Early on the Guest hasn't set the IDT entries (or maybe it put a
 352         * bogus one in): if we fail here, the Guest will be killed.
 353         */
 354        if (!idt_present(cpu->arch.idt[num].a, cpu->arch.idt[num].b))
 355                return false;
 356        set_guest_interrupt(cpu, cpu->arch.idt[num].a,
 357                            cpu->arch.idt[num].b, has_err(num));
 358        return true;
 359}
 360
 361/*H:250
 362 * Here's the hard part: returning to the Host every time a trap happens
 363 * and then calling deliver_trap() and re-entering the Guest is slow.
 364 * Particularly because Guest userspace system calls are traps (usually trap
 365 * 128).
 366 *
 367 * So we'd like to set up the IDT to tell the CPU to deliver traps directly
 368 * into the Guest.  This is possible, but the complexities cause the size of
 369 * this file to double!  However, 150 lines of code is worth writing for taking
 370 * system calls down from 1750ns to 270ns.  Plus, if lguest didn't do it, all
 371 * the other hypervisors would beat it up at lunchtime.
 372 *
 373 * This routine indicates if a particular trap number could be delivered
 374 * directly.
 375 */
 376static bool direct_trap(unsigned int num)
 377{
 378        /*
 379         * Hardware interrupts don't go to the Guest at all (except system
 380         * call).
 381         */
 382        if (num >= FIRST_EXTERNAL_VECTOR && !could_be_syscall(num))
 383                return false;
 384
 385        /*
 386         * The Host needs to see page faults (for shadow paging and to save the
 387         * fault address), general protection faults (in/out emulation) and
 388         * device not available (TS handling) and of course, the hypercall trap.
 389         */
 390        return num != 14 && num != 13 && num != 7 && num != LGUEST_TRAP_ENTRY;
 391}
 392/*:*/
 393
 394/*M:005
 395 * The Guest has the ability to turn its interrupt gates into trap gates,
 396 * if it is careful.  The Host will let trap gates can go directly to the
 397 * Guest, but the Guest needs the interrupts atomically disabled for an
 398 * interrupt gate.  It can do this by pointing the trap gate at instructions
 399 * within noirq_start and noirq_end, where it can safely disable interrupts.
 400 */
 401
 402/*M:006
 403 * The Guests do not use the sysenter (fast system call) instruction,
 404 * because it's hardcoded to enter privilege level 0 and so can't go direct.
 405 * It's about twice as fast as the older "int 0x80" system call, so it might
 406 * still be worthwhile to handle it in the Switcher and lcall down to the
 407 * Guest.  The sysenter semantics are hairy tho: search for that keyword in
 408 * entry.S
 409:*/
 410
 411/*H:260
 412 * When we make traps go directly into the Guest, we need to make sure
 413 * the kernel stack is valid (ie. mapped in the page tables).  Otherwise, the
 414 * CPU trying to deliver the trap will fault while trying to push the interrupt
 415 * words on the stack: this is called a double fault, and it forces us to kill
 416 * the Guest.
 417 *
 418 * Which is deeply unfair, because (literally!) it wasn't the Guests' fault.
 419 */
 420void pin_stack_pages(struct lg_cpu *cpu)
 421{
 422        unsigned int i;
 423
 424        /*
 425         * Depending on the CONFIG_4KSTACKS option, the Guest can have one or
 426         * two pages of stack space.
 427         */
 428        for (i = 0; i < cpu->lg->stack_pages; i++)
 429                /*
 430                 * The stack grows *upwards*, so the address we're given is the
 431                 * start of the page after the kernel stack.  Subtract one to
 432                 * get back onto the first stack page, and keep subtracting to
 433                 * get to the rest of the stack pages.
 434                 */
 435                pin_page(cpu, cpu->esp1 - 1 - i * PAGE_SIZE);
 436}
 437
 438/*
 439 * Direct traps also mean that we need to know whenever the Guest wants to use
 440 * a different kernel stack, so we can change the guest TSS to use that
 441 * stack.  The TSS entries expect a virtual address, so unlike most addresses
 442 * the Guest gives us, the "esp" (stack pointer) value here is virtual, not
 443 * physical.
 444 *
 445 * In Linux each process has its own kernel stack, so this happens a lot: we
 446 * change stacks on each context switch.
 447 */
 448void guest_set_stack(struct lg_cpu *cpu, u32 seg, u32 esp, unsigned int pages)
 449{
 450        /*
 451         * You're not allowed a stack segment with privilege level 0: bad Guest!
 452         */
 453        if ((seg & 0x3) != GUEST_PL)
 454                kill_guest(cpu, "bad stack segment %i", seg);
 455        /* We only expect one or two stack pages. */
 456        if (pages > 2)
 457                kill_guest(cpu, "bad stack pages %u", pages);
 458        /* Save where the stack is, and how many pages */
 459        cpu->ss1 = seg;
 460        cpu->esp1 = esp;
 461        cpu->lg->stack_pages = pages;
 462        /* Make sure the new stack pages are mapped */
 463        pin_stack_pages(cpu);
 464}
 465
 466/*
 467 * All this reference to mapping stacks leads us neatly into the other complex
 468 * part of the Host: page table handling.
 469 */
 470
 471/*H:235
 472 * This is the routine which actually checks the Guest's IDT entry and
 473 * transfers it into the entry in "struct lguest":
 474 */
 475static void set_trap(struct lg_cpu *cpu, struct desc_struct *trap,
 476                     unsigned int num, u32 lo, u32 hi)
 477{
 478        u8 type = idt_type(lo, hi);
 479
 480        /* We zero-out a not-present entry */
 481        if (!idt_present(lo, hi)) {
 482                trap->a = trap->b = 0;
 483                return;
 484        }
 485
 486        /* We only support interrupt and trap gates. */
 487        if (type != 0xE && type != 0xF)
 488                kill_guest(cpu, "bad IDT type %i", type);
 489
 490        /*
 491         * We only copy the handler address, present bit, privilege level and
 492         * type.  The privilege level controls where the trap can be triggered
 493         * manually with an "int" instruction.  This is usually GUEST_PL,
 494         * except for system calls which userspace can use.
 495         */
 496        trap->a = ((__KERNEL_CS|GUEST_PL)<<16) | (lo&0x0000FFFF);
 497        trap->b = (hi&0xFFFFEF00);
 498}
 499
 500/*H:230
 501 * While we're here, dealing with delivering traps and interrupts to the
 502 * Guest, we might as well complete the picture: how the Guest tells us where
 503 * it wants them to go.  This would be simple, except making traps fast
 504 * requires some tricks.
 505 *
 506 * We saw the Guest setting Interrupt Descriptor Table (IDT) entries with the
 507 * LHCALL_LOAD_IDT_ENTRY hypercall before: that comes here.
 508 */
 509void load_guest_idt_entry(struct lg_cpu *cpu, unsigned int num, u32 lo, u32 hi)
 510{
 511        /*
 512         * Guest never handles: NMI, doublefault, spurious interrupt or
 513         * hypercall.  We ignore when it tries to set them.
 514         */
 515        if (num == 2 || num == 8 || num == 15 || num == LGUEST_TRAP_ENTRY)
 516                return;
 517
 518        /*
 519         * Mark the IDT as changed: next time the Guest runs we'll know we have
 520         * to copy this again.
 521         */
 522        cpu->changed |= CHANGED_IDT;
 523
 524        /* Check that the Guest doesn't try to step outside the bounds. */
 525        if (num >= ARRAY_SIZE(cpu->arch.idt))
 526                kill_guest(cpu, "Setting idt entry %u", num);
 527        else
 528                set_trap(cpu, &cpu->arch.idt[num], num, lo, hi);
 529}
 530
 531/*
 532 * The default entry for each interrupt points into the Switcher routines which
 533 * simply return to the Host.  The run_guest() loop will then call
 534 * deliver_trap() to bounce it back into the Guest.
 535 */
 536static void default_idt_entry(struct desc_struct *idt,
 537                              int trap,
 538                              const unsigned long handler,
 539                              const struct desc_struct *base)
 540{
 541        /* A present interrupt gate. */
 542        u32 flags = 0x8e00;
 543
 544        /*
 545         * Set the privilege level on the entry for the hypercall: this allows
 546         * the Guest to use the "int" instruction to trigger it.
 547         */
 548        if (trap == LGUEST_TRAP_ENTRY)
 549                flags |= (GUEST_PL << 13);
 550        else if (base)
 551                /*
 552                 * Copy privilege level from what Guest asked for.  This allows
 553                 * debug (int 3) traps from Guest userspace, for example.
 554                 */
 555                flags |= (base->b & 0x6000);
 556
 557        /* Now pack it into the IDT entry in its weird format. */
 558        idt->a = (LGUEST_CS<<16) | (handler&0x0000FFFF);
 559        idt->b = (handler&0xFFFF0000) | flags;
 560}
 561
 562/* When the Guest first starts, we put default entries into the IDT. */
 563void setup_default_idt_entries(struct lguest_ro_state *state,
 564                               const unsigned long *def)
 565{
 566        unsigned int i;
 567
 568        for (i = 0; i < ARRAY_SIZE(state->guest_idt); i++)
 569                default_idt_entry(&state->guest_idt[i], i, def[i], NULL);
 570}
 571
 572/*H:240
 573 * We don't use the IDT entries in the "struct lguest" directly, instead
 574 * we copy them into the IDT which we've set up for Guests on this CPU, just
 575 * before we run the Guest.  This routine does that copy.
 576 */
 577void copy_traps(const struct lg_cpu *cpu, struct desc_struct *idt,
 578                const unsigned long *def)
 579{
 580        unsigned int i;
 581
 582        /*
 583         * We can simply copy the direct traps, otherwise we use the default
 584         * ones in the Switcher: they will return to the Host.
 585         */
 586        for (i = 0; i < ARRAY_SIZE(cpu->arch.idt); i++) {
 587                const struct desc_struct *gidt = &cpu->arch.idt[i];
 588
 589                /* If no Guest can ever override this trap, leave it alone. */
 590                if (!direct_trap(i))
 591                        continue;
 592
 593                /*
 594                 * Only trap gates (type 15) can go direct to the Guest.
 595                 * Interrupt gates (type 14) disable interrupts as they are
 596                 * entered, which we never let the Guest do.  Not present
 597                 * entries (type 0x0) also can't go direct, of course.
 598                 *
 599                 * If it can't go direct, we still need to copy the priv. level:
 600                 * they might want to give userspace access to a software
 601                 * interrupt.
 602                 */
 603                if (idt_type(gidt->a, gidt->b) == 0xF)
 604                        idt[i] = *gidt;
 605                else
 606                        default_idt_entry(&idt[i], i, def[i], gidt);
 607        }
 608}
 609
 610/*H:200
 611 * The Guest Clock.
 612 *
 613 * There are two sources of virtual interrupts.  We saw one in lguest_user.c:
 614 * the Launcher sending interrupts for virtual devices.  The other is the Guest
 615 * timer interrupt.
 616 *
 617 * The Guest uses the LHCALL_SET_CLOCKEVENT hypercall to tell us how long to
 618 * the next timer interrupt (in nanoseconds).  We use the high-resolution timer
 619 * infrastructure to set a callback at that time.
 620 *
 621 * 0 means "turn off the clock".
 622 */
 623void guest_set_clockevent(struct lg_cpu *cpu, unsigned long delta)
 624{
 625        ktime_t expires;
 626
 627        if (unlikely(delta == 0)) {
 628                /* Clock event device is shutting down. */
 629                hrtimer_cancel(&cpu->hrt);
 630                return;
 631        }
 632
 633        /*
 634         * We use wallclock time here, so the Guest might not be running for
 635         * all the time between now and the timer interrupt it asked for.  This
 636         * is almost always the right thing to do.
 637         */
 638        expires = ktime_add_ns(ktime_get_real(), delta);
 639        hrtimer_start(&cpu->hrt, expires, HRTIMER_MODE_ABS);
 640}
 641
 642/* This is the function called when the Guest's timer expires. */
 643static enum hrtimer_restart clockdev_fn(struct hrtimer *timer)
 644{
 645        struct lg_cpu *cpu = container_of(timer, struct lg_cpu, hrt);
 646
 647        /* Remember the first interrupt is the timer interrupt. */
 648        set_interrupt(cpu, 0);
 649        return HRTIMER_NORESTART;
 650}
 651
 652/* This sets up the timer for this Guest. */
 653void init_clockdev(struct lg_cpu *cpu)
 654{
 655        hrtimer_init(&cpu->hrt, CLOCK_REALTIME, HRTIMER_MODE_ABS);
 656        cpu->hrt.function = clockdev_fn;
 657}
 658
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