cpu-exec.c

   1 /*
   2  *  emulator main execution loop
   3  *
   4  *  Copyright (c) 2003-2005 Fabrice Bellard
   5  *
   6  * This library is free software; you can redistribute it and/or
   7  * modify it under the terms of the GNU Lesser General Public
   8  * License as published by the Free Software Foundation; either
   9  * version 2 of the License, or (at your option) any later version.
  10  *
  11  * This library is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14  * Lesser General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU Lesser General Public
  17  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  18  */
  19 #include "qemu/osdep.h"
  20 #include "cpu.h"
  21 #include "trace.h"
  22 #include "disas/disas.h"
  23 #include "exec/exec-all.h"
  24 #include "tcg.h"
  25 #include "qemu/atomic.h"
  26 #include "sysemu/qtest.h"
  27 #include "qemu/timer.h"
  28 #include "exec/address-spaces.h"
  29 #include "qemu/rcu.h"
  30 #include "exec/tb-hash.h"
  31 #include "exec/log.h"
  32 #if defined(TARGET_I386) && !defined(CONFIG_USER_ONLY)
  33 #include "hw/i386/apic.h"
  34 #endif
  35 #include "sysemu/replay.h"
  36
  37 /* -icount align implementation. */
  38
  39 typedef struct SyncClocks {
  40     int64_t diff_clk;
  41     int64_t last_cpu_icount;
  42     int64_t realtime_clock;
  43 } SyncClocks;
  44
  45 #if !defined(CONFIG_USER_ONLY)
  46 /* Allow the guest to have a max 3ms advance.
  47  * The difference between the 2 clocks could therefore
  48  * oscillate around 0.
  49  */
  50 #define VM_CLOCK_ADVANCE 3000000
  51 #define THRESHOLD_REDUCE 1.5
  52 #define MAX_DELAY_PRINT_RATE 2000000000LL
  53 #define MAX_NB_PRINTS 100
  54
  55 static void align_clocks(SyncClocks *sc, const CPUState *cpu)
  56 {
  57     int64_t cpu_icount;
  58
  59     if (!icount_align_option) {
  60         return;
  61     }
  62
  63     cpu_icount = cpu->icount_extra + cpu->icount_decr.u16.low;
  64     sc->diff_clk += cpu_icount_to_ns(sc->last_cpu_icount - cpu_icount);
  65     sc->last_cpu_icount = cpu_icount;
  66
  67     if (sc->diff_clk > VM_CLOCK_ADVANCE) {
  68 #ifndef _WIN32
  69         struct timespec sleep_delay, rem_delay;
  70         sleep_delay.tv_sec = sc->diff_clk / 1000000000LL;
  71         sleep_delay.tv_nsec = sc->diff_clk % 1000000000LL;
  72         if (nanosleep(&sleep_delay, &rem_delay) < 0) {
  73             sc->diff_clk = rem_delay.tv_sec * 1000000000LL + rem_delay.tv_nsec;
  74         } else {
  75             sc->diff_clk = 0;
  76         }
  77 #else
  78         Sleep(sc->diff_clk / SCALE_MS);
  79         sc->diff_clk = 0;
  80 #endif
  81     }
  82 }
  83
  84 static void print_delay(const SyncClocks *sc)
  85 {
  86     static float threshold_delay;
  87     static int64_t last_realtime_clock;
  88     static int nb_prints;
  89
  90     if (icount_align_option &&
  91         sc->realtime_clock - last_realtime_clock >= MAX_DELAY_PRINT_RATE &&
  92         nb_prints < MAX_NB_PRINTS) {
  93         if ((-sc->diff_clk / (float)1000000000LL > threshold_delay) ||
  94             (-sc->diff_clk / (float)1000000000LL <
  95              (threshold_delay - THRESHOLD_REDUCE))) {
  96             threshold_delay = (-sc->diff_clk / 1000000000LL) + 1;
  97             printf("Warning: The guest is now late by %.1f to %.1f seconds\n",
  98                    threshold_delay - 1,
  99                    threshold_delay);
 100             nb_prints++;
 101             last_realtime_clock = sc->realtime_clock;
 102         }
 103     }
 104 }
 105
 106 static void init_delay_params(SyncClocks *sc,
 107                               const CPUState *cpu)
 108 {
 109     if (!icount_align_option) {
 110         return;
 111     }
 112     sc->realtime_clock = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL_RT);
 113     sc->diff_clk = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) - sc->realtime_clock;
 114     sc->last_cpu_icount = cpu->icount_extra + cpu->icount_decr.u16.low;
 115     if (sc->diff_clk < max_delay) {
 116         max_delay = sc->diff_clk;
 117     }
 118     if (sc->diff_clk > max_advance) {
 119         max_advance = sc->diff_clk;
 120     }
 121
 122     /* Print every 2s max if the guest is late. We limit the number
 123        of printed messages to NB_PRINT_MAX(currently 100) */
 124     print_delay(sc);
 125 }
 126 #else
 127 static void align_clocks(SyncClocks *sc, const CPUState *cpu)
 128 {
 129 }
 130
 131 static void init_delay_params(SyncClocks *sc, const CPUState *cpu)
 132 {
 133 }
 134 #endif /* CONFIG USER ONLY */
 135
 136 /* Execute a TB, and fix up the CPU state afterwards if necessary */
 137 static inline tcg_target_ulong cpu_tb_exec(CPUState *cpu, TranslationBlock *itb)
 138 {
 139     CPUArchState *env = cpu->env_ptr;
 140     uintptr_t ret;
 141     TranslationBlock *last_tb;
 142     int tb_exit;
 143     uint8_t *tb_ptr = itb->tc_ptr;
 144
 145     qemu_log_mask_and_addr(CPU_LOG_EXEC, itb->pc,
 146                            "Trace %p [" TARGET_FMT_lx "] %s\n",
 147                            itb->tc_ptr, itb->pc, lookup_symbol(itb->pc));
 148
 149 #if defined(DEBUG_DISAS)
 150     if (qemu_loglevel_mask(CPU_LOG_TB_CPU)
 151         && qemu_log_in_addr_range(itb->pc)) {
 152 #if defined(TARGET_I386)
 153         log_cpu_state(cpu, CPU_DUMP_CCOP);
 154 #elif defined(TARGET_M68K)
 155         /* ??? Should not modify env state for dumping.  */
 156         cpu_m68k_flush_flags(env, env->cc_op);
 157         env->cc_op = CC_OP_FLAGS;
 158         env->sr = (env->sr & 0xffe0) | env->cc_dest | (env->cc_x << 4);
 159         log_cpu_state(cpu, 0);
 160 #else
 161         log_cpu_state(cpu, 0);
 162 #endif
 163     }
 164 #endif /* DEBUG_DISAS */
 165
 166     cpu->can_do_io = !use_icount;
 167     ret = tcg_qemu_tb_exec(env, tb_ptr);
 168     cpu->can_do_io = 1;
 169     last_tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
 170     tb_exit = ret & TB_EXIT_MASK;
 171     trace_exec_tb_exit(last_tb, tb_exit);
 172
 173     if (tb_exit > TB_EXIT_IDX1) {
 174         /* We didn't start executing this TB (eg because the instruction
 175          * counter hit zero); we must restore the guest PC to the address
 176          * of the start of the TB.
 177          */
 178         CPUClass *cc = CPU_GET_CLASS(cpu);
 179         qemu_log_mask_and_addr(CPU_LOG_EXEC, last_tb->pc,
 180                                "Stopped execution of TB chain before %p ["
 181                                TARGET_FMT_lx "] %s\n",
 182                                last_tb->tc_ptr, last_tb->pc,
 183                                lookup_symbol(last_tb->pc));
 184         if (cc->synchronize_from_tb) {
 185             cc->synchronize_from_tb(cpu, last_tb);
 186         } else {
 187             assert(cc->set_pc);
 188             cc->set_pc(cpu, last_tb->pc);
 189         }
 190     }
 191     if (tb_exit == TB_EXIT_REQUESTED) {
 192         /* We were asked to stop executing TBs (probably a pending
 193          * interrupt. We've now stopped, so clear the flag.
 194          */
 195         atomic_set(&cpu->tcg_exit_req, 0);
 196     }
 197     return ret;
 198 }
 199
 200 #ifndef CONFIG_USER_ONLY
 201 /* Execute the code without caching the generated code. An interpreter
 202    could be used if available. */
 203 static void cpu_exec_nocache(CPUState *cpu, int max_cycles,
 204                              TranslationBlock *orig_tb, bool ignore_icount)
 205 {
 206     TranslationBlock *tb;
 207
 208     /* Should never happen.
 209        We only end up here when an existing TB is too long.  */
 210     if (max_cycles > CF_COUNT_MASK)
 211         max_cycles = CF_COUNT_MASK;
 212
 213     tb = tb_gen_code(cpu, orig_tb->pc, orig_tb->cs_base, orig_tb->flags,
 214                      max_cycles | CF_NOCACHE
 215                          | (ignore_icount ? CF_IGNORE_ICOUNT : 0));
 216     tb->orig_tb = orig_tb;
 217     /* execute the generated code */
 218     trace_exec_tb_nocache(tb, tb->pc);
 219     cpu_tb_exec(cpu, tb);
 220     tb_phys_invalidate(tb, -1);
 221     tb_free(tb);
 222 }
 223 #endif
 224
 225 struct tb_desc {
 226     target_ulong pc;
 227     target_ulong cs_base;
 228     CPUArchState *env;
 229     tb_page_addr_t phys_page1;
 230     uint32_t flags;
 231 };
 232
 233 static bool tb_cmp(const void *p, const void *d)
 234 {
 235     const TranslationBlock *tb = p;
 236     const struct tb_desc *desc = d;
 237
 238     if (tb->pc == desc->pc &&
 239         tb->page_addr[0] == desc->phys_page1 &&
 240         tb->cs_base == desc->cs_base &&
 241         tb->flags == desc->flags &&
 242         !atomic_read(&tb->invalid)) {
 243         /* check next page if needed */
 244         if (tb->page_addr[1] == -1) {
 245             return true;
 246         } else {
 247             tb_page_addr_t phys_page2;
 248             target_ulong virt_page2;
 249
 250             virt_page2 = (desc->pc & TARGET_PAGE_MASK) + TARGET_PAGE_SIZE;
 251             phys_page2 = get_page_addr_code(desc->env, virt_page2);
 252             if (tb->page_addr[1] == phys_page2) {
 253                 return true;
 254             }
 255         }
 256     }
 257     return false;
 258 }
 259
 260 static TranslationBlock *tb_htable_lookup(CPUState *cpu,
 261                                           target_ulong pc,
 262                                           target_ulong cs_base,
 263                                           uint32_t flags)
 264 {
 265     tb_page_addr_t phys_pc;
 266     struct tb_desc desc;
 267     uint32_t h;
 268
 269     desc.env = (CPUArchState *)cpu->env_ptr;
 270     desc.cs_base = cs_base;
 271     desc.flags = flags;
 272     desc.pc = pc;
 273     phys_pc = get_page_addr_code(desc.env, pc);
 274     desc.phys_page1 = phys_pc & TARGET_PAGE_MASK;
 275     h = tb_hash_func(phys_pc, pc, flags);
 276     return qht_lookup(&tcg_ctx.tb_ctx.htable, tb_cmp, &desc, h);
 277 }
 278
 279 static inline TranslationBlock *tb_find(CPUState *cpu,
 280                                         TranslationBlock *last_tb,
 281                                         int tb_exit)
 282 {
 283     CPUArchState *env = (CPUArchState *)cpu->env_ptr;
 284     TranslationBlock *tb;
 285     target_ulong cs_base, pc;
 286     uint32_t flags;
 287     bool have_tb_lock = false;
 288
 289     /* we record a subset of the CPU state. It will
 290        always be the same before a given translated block
 291        is executed. */
 292     cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
 293     tb = atomic_rcu_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)]);
 294     if (unlikely(!tb || tb->pc != pc || tb->cs_base != cs_base ||
 295                  tb->flags != flags)) {
 296         tb = tb_htable_lookup(cpu, pc, cs_base, flags);
 297         if (!tb) {
 298
 299             /* mmap_lock is needed by tb_gen_code, and mmap_lock must be
 300              * taken outside tb_lock. As system emulation is currently
 301              * single threaded the locks are NOPs.
 302              */
 303             mmap_lock();
 304             tb_lock();
 305             have_tb_lock = true;
 306
 307             /* There's a chance that our desired tb has been translated while
 308              * taking the locks so we check again inside the lock.
 309              */
 310             tb = tb_htable_lookup(cpu, pc, cs_base, flags);
 311             if (!tb) {
 312                 /* if no translated code available, then translate it now */
 313                 tb = tb_gen_code(cpu, pc, cs_base, flags, 0);
 314             }
 315
 316             mmap_unlock();
 317         }
 318
 319         /* We add the TB in the virtual pc hash table for the fast lookup */
 320         atomic_set(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)], tb);
 321     }
 322 #ifndef CONFIG_USER_ONLY
 323     /* We don't take care of direct jumps when address mapping changes in
 324      * system emulation. So it's not safe to make a direct jump to a TB
 325      * spanning two pages because the mapping for the second page can change.
 326      */
 327     if (tb->page_addr[1] != -1) {
 328         last_tb = NULL;
 329     }
 330 #endif
 331     /* See if we can patch the calling TB. */
 332     if (last_tb && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
 333         if (!have_tb_lock) {
 334             tb_lock();
 335             have_tb_lock = true;
 336         }
 337         if (!tb->invalid) {
 338             tb_add_jump(last_tb, tb_exit, tb);
 339         }
 340     }
 341     if (have_tb_lock) {
 342         tb_unlock();
 343     }
 344     return tb;
 345 }
 346
 347 static inline bool cpu_handle_halt(CPUState *cpu)
 348 {
 349     if (cpu->halted) {
 350 #if defined(TARGET_I386) && !defined(CONFIG_USER_ONLY)
 351         if ((cpu->interrupt_request & CPU_INTERRUPT_POLL)
 352             && replay_interrupt()) {
 353             X86CPU *x86_cpu = X86_CPU(cpu);
 354             apic_poll_irq(x86_cpu->apic_state);
 355             cpu_reset_interrupt(cpu, CPU_INTERRUPT_POLL);
 356         }
 357 #endif
 358         if (!cpu_has_work(cpu)) {
 359             current_cpu = NULL;
 360             return true;
 361         }
 362
 363         cpu->halted = 0;
 364     }
 365
 366     return false;
 367 }
 368
 369 static inline void cpu_handle_debug_exception(CPUState *cpu)
 370 {
 371     CPUClass *cc = CPU_GET_CLASS(cpu);
 372     CPUWatchpoint *wp;
 373
 374     if (!cpu->watchpoint_hit) {
 375         QTAILQ_FOREACH(wp, &cpu->watchpoints, entry) {
 376             wp->flags &= ~BP_WATCHPOINT_HIT;
 377         }
 378     }
 379
 380     cc->debug_excp_handler(cpu);
 381 }
 382
 383 static inline bool cpu_handle_exception(CPUState *cpu, int *ret)
 384 {
 385     if (cpu->exception_index >= 0) {
 386         if (cpu->exception_index >= EXCP_INTERRUPT) {
 387             /* exit request from the cpu execution loop */
 388             *ret = cpu->exception_index;
 389             if (*ret == EXCP_DEBUG) {
 390                 cpu_handle_debug_exception(cpu);
 391             }
 392             cpu->exception_index = -1;
 393             return true;
 394         } else {
 395 #if defined(CONFIG_USER_ONLY)
 396             /* if user mode only, we simulate a fake exception
 397                which will be handled outside the cpu execution
 398                loop */
 399 #if defined(TARGET_I386)
 400             CPUClass *cc = CPU_GET_CLASS(cpu);
 401             cc->do_interrupt(cpu);
 402 #endif
 403             *ret = cpu->exception_index;
 404             cpu->exception_index = -1;
 405             return true;
 406 #else
 407             if (replay_exception()) {
 408                 CPUClass *cc = CPU_GET_CLASS(cpu);
 409                 cc->do_interrupt(cpu);
 410                 cpu->exception_index = -1;
 411             } else if (!replay_has_interrupt()) {
 412                 /* give a chance to iothread in replay mode */
 413                 *ret = EXCP_INTERRUPT;
 414                 return true;
 415             }
 416 #endif
 417         }
 418 #ifndef CONFIG_USER_ONLY
 419     } else if (replay_has_exception()
 420                && cpu->icount_decr.u16.low + cpu->icount_extra == 0) {
 421         /* try to cause an exception pending in the log */
 422         cpu_exec_nocache(cpu, 1, tb_find(cpu, NULL, 0), true);
 423         *ret = -1;
 424         return true;
 425 #endif
 426     }
 427
 428     return false;
 429 }
 430
 431 static inline void cpu_handle_interrupt(CPUState *cpu,
 432                                         TranslationBlock **last_tb)
 433 {
 434     CPUClass *cc = CPU_GET_CLASS(cpu);
 435     int interrupt_request = cpu->interrupt_request;
 436
 437     if (unlikely(interrupt_request)) {
 438         if (unlikely(cpu->singlestep_enabled & SSTEP_NOIRQ)) {
 439             /* Mask out external interrupts for this step. */
 440             interrupt_request &= ~CPU_INTERRUPT_SSTEP_MASK;
 441         }
 442         if (interrupt_request & CPU_INTERRUPT_DEBUG) {
 443             cpu->interrupt_request &= ~CPU_INTERRUPT_DEBUG;
 444             cpu->exception_index = EXCP_DEBUG;
 445             cpu_loop_exit(cpu);
 446         }
 447         if (replay_mode == REPLAY_MODE_PLAY && !replay_has_interrupt()) {
 448             /* Do nothing */
 449         } else if (interrupt_request & CPU_INTERRUPT_HALT) {
 450             replay_interrupt();
 451             cpu->interrupt_request &= ~CPU_INTERRUPT_HALT;
 452             cpu->halted = 1;
 453             cpu->exception_index = EXCP_HLT;
 454             cpu_loop_exit(cpu);
 455         }
 456 #if defined(TARGET_I386)
 457         else if (interrupt_request & CPU_INTERRUPT_INIT) {
 458             X86CPU *x86_cpu = X86_CPU(cpu);
 459             CPUArchState *env = &x86_cpu->env;
 460             replay_interrupt();
 461             cpu_svm_check_intercept_param(env, SVM_EXIT_INIT, 0);
 462             do_cpu_init(x86_cpu);
 463             cpu->exception_index = EXCP_HALTED;
 464             cpu_loop_exit(cpu);
 465         }
 466 #else
 467         else if (interrupt_request & CPU_INTERRUPT_RESET) {
 468             replay_interrupt();
 469             cpu_reset(cpu);
 470             cpu_loop_exit(cpu);
 471         }
 472 #endif
 473         /* The target hook has 3 exit conditions:
 474            False when the interrupt isn't processed,
 475            True when it is, and we should restart on a new TB,
 476            and via longjmp via cpu_loop_exit.  */
 477         else {
 478             replay_interrupt();
 479             if (cc->cpu_exec_interrupt(cpu, interrupt_request)) {
 480                 *last_tb = NULL;
 481             }
 482             /* The target hook may have updated the 'cpu->interrupt_request';
 483              * reload the 'interrupt_request' value */
 484             interrupt_request = cpu->interrupt_request;
 485         }
 486         if (interrupt_request & CPU_INTERRUPT_EXITTB) {
 487             cpu->interrupt_request &= ~CPU_INTERRUPT_EXITTB;
 488             /* ensure that no TB jump will be modified as
 489                the program flow was changed */
 490             *last_tb = NULL;
 491         }
 492     }
 493     if (unlikely(atomic_read(&cpu->exit_request) || replay_has_interrupt())) {
 494         atomic_set(&cpu->exit_request, 0);
 495         cpu->exception_index = EXCP_INTERRUPT;
 496         cpu_loop_exit(cpu);
 497     }
 498 }
 499
 500 static inline void cpu_loop_exec_tb(CPUState *cpu, TranslationBlock *tb,
 501                                     TranslationBlock **last_tb, int *tb_exit,
 502                                     SyncClocks *sc)
 503 {
 504     uintptr_t ret;
 505
 506     if (unlikely(atomic_read(&cpu->exit_request))) {
 507         return;
 508     }
 509
 510     trace_exec_tb(tb, tb->pc);
 511     ret = cpu_tb_exec(cpu, tb);
 512     *last_tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
 513     *tb_exit = ret & TB_EXIT_MASK;
 514     switch (*tb_exit) {
 515     case TB_EXIT_REQUESTED:
 516         /* Something asked us to stop executing
 517          * chained TBs; just continue round the main
 518          * loop. Whatever requested the exit will also
 519          * have set something else (eg exit_request or
 520          * interrupt_request) which we will handle
 521          * next time around the loop.  But we need to
 522          * ensure the tcg_exit_req read in generated code
 523          * comes before the next read of cpu->exit_request
 524          * or cpu->interrupt_request.
 525          */
 526         smp_rmb();
 527         *last_tb = NULL;
 528         break;
 529     case TB_EXIT_ICOUNT_EXPIRED:
 530     {
 531         /* Instruction counter expired.  */
 532 #ifdef CONFIG_USER_ONLY
 533         abort();
 534 #else
 535         int insns_left = cpu->icount_decr.u32;
 536         if (cpu->icount_extra && insns_left >= 0) {
 537             /* Refill decrementer and continue execution.  */
 538             cpu->icount_extra += insns_left;
 539             insns_left = MIN(0xffff, cpu->icount_extra);
 540             cpu->icount_extra -= insns_left;
 541             cpu->icount_decr.u16.low = insns_left;
 542         } else {
 543             if (insns_left > 0) {
 544                 /* Execute remaining instructions.  */
 545                 cpu_exec_nocache(cpu, insns_left, *last_tb, false);
 546                 align_clocks(sc, cpu);
 547             }
 548             cpu->exception_index = EXCP_INTERRUPT;
 549             *last_tb = NULL;
 550             cpu_loop_exit(cpu);
 551         }
 552         break;
 553 #endif
 554     }
 555     default:
 556         break;
 557     }
 558 }
 559
 560 /* main execution loop */
 561
 562 int cpu_exec(CPUState *cpu)
 563 {
 564     CPUClass *cc = CPU_GET_CLASS(cpu);
 565     int ret;
 566     SyncClocks sc;
 567
 568     /* replay_interrupt may need current_cpu */
 569     current_cpu = cpu;
 570
 571     if (cpu_handle_halt(cpu)) {
 572         return EXCP_HALTED;
 573     }
 574
 575     atomic_mb_set(&tcg_current_cpu, cpu);
 576     rcu_read_lock();
 577
 578     if (unlikely(atomic_mb_read(&exit_request))) {
 579         cpu->exit_request = 1;
 580     }
 581
 582     cc->cpu_exec_enter(cpu);
 583
 584     /* Calculate difference between guest clock and host clock.
 585      * This delay includes the delay of the last cycle, so
 586      * what we have to do is sleep until it is 0. As for the
 587      * advance/delay we gain here, we try to fix it next time.
 588      */
 589     init_delay_params(&sc, cpu);
 590
 591     for(;;) {
 592         /* prepare setjmp context for exception handling */
 593         if (sigsetjmp(cpu->jmp_env, 0) == 0) {
 594             TranslationBlock *tb, *last_tb = NULL;
 595             int tb_exit = 0;
 596
 597             /* if an exception is pending, we execute it here */
 598             if (cpu_handle_exception(cpu, &ret)) {
 599                 break;
 600             }
 601
 602             for(;;) {
 603                 cpu_handle_interrupt(cpu, &last_tb);
 604                 tb = tb_find(cpu, last_tb, tb_exit);
 605                 cpu_loop_exec_tb(cpu, tb, &last_tb, &tb_exit, &sc);
 606                 /* Try to align the host and virtual clocks
 607                    if the guest is in advance */
 608                 align_clocks(&sc, cpu);
 609             } /* for(;;) */
 610         } else {
 611 #if defined(__clang__) || !QEMU_GNUC_PREREQ(4, 6)
 612             /* Some compilers wrongly smash all local variables after
 613              * siglongjmp. There were bug reports for gcc 4.5.0 and clang.
 614              * Reload essential local variables here for those compilers.
 615              * Newer versions of gcc would complain about this code (-Wclobbered). */
 616             cpu = current_cpu;
 617             cc = CPU_GET_CLASS(cpu);
 618 #else /* buggy compiler */
 619             /* Assert that the compiler does not smash local variables. */
 620             g_assert(cpu == current_cpu);
 621             g_assert(cc == CPU_GET_CLASS(cpu));
 622 #endif /* buggy compiler */
 623             cpu->can_do_io = 1;
 624             tb_lock_reset();
 625         }
 626     } /* for(;;) */
 627
 628     cc->cpu_exec_exit(cpu);
 629     rcu_read_unlock();
 630
 631     /* fail safe : never use current_cpu outside cpu_exec() */
 632     current_cpu = NULL;
 633
 634     /* Does not need atomic_mb_set because a spurious wakeup is okay.  */
 635     atomic_set(&tcg_current_cpu, NULL);
 636     return ret;
 637 }