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 #if defined(TARGET_I386)
 152         log_cpu_state(cpu, CPU_DUMP_CCOP);
 153 #elif defined(TARGET_M68K)
 154         /* ??? Should not modify env state for dumping.  */
 155         cpu_m68k_flush_flags(env, env->cc_op);
 156         env->cc_op = CC_OP_FLAGS;
 157         env->sr = (env->sr & 0xffe0) | env->cc_dest | (env->cc_x << 4);
 158         log_cpu_state(cpu, 0);
 159 #else
 160         log_cpu_state(cpu, 0);
 161 #endif
 162     }
 163 #endif /* DEBUG_DISAS */
 164
 165     cpu->can_do_io = !use_icount;
 166     ret = tcg_qemu_tb_exec(env, tb_ptr);
 167     cpu->can_do_io = 1;
 168     last_tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
 169     tb_exit = ret & TB_EXIT_MASK;
 170     trace_exec_tb_exit(last_tb, tb_exit);
 171
 172     if (tb_exit > TB_EXIT_IDX1) {
 173         /* We didn't start executing this TB (eg because the instruction
 174          * counter hit zero); we must restore the guest PC to the address
 175          * of the start of the TB.
 176          */
 177         CPUClass *cc = CPU_GET_CLASS(cpu);
 178         qemu_log_mask_and_addr(CPU_LOG_EXEC, last_tb->pc,
 179                                "Stopped execution of TB chain before %p ["
 180                                TARGET_FMT_lx "] %s\n",
 181                                last_tb->tc_ptr, last_tb->pc,
 182                                lookup_symbol(last_tb->pc));
 183         if (cc->synchronize_from_tb) {
 184             cc->synchronize_from_tb(cpu, last_tb);
 185         } else {
 186             assert(cc->set_pc);
 187             cc->set_pc(cpu, last_tb->pc);
 188         }
 189     }
 190     if (tb_exit == TB_EXIT_REQUESTED) {
 191         /* We were asked to stop executing TBs (probably a pending
 192          * interrupt. We've now stopped, so clear the flag.
 193          */
 194         cpu->tcg_exit_req = 0;
 195     }
 196     return ret;
 197 }
 198
 199 #ifndef CONFIG_USER_ONLY
 200 /* Execute the code without caching the generated code. An interpreter
 201    could be used if available. */
 202 static void cpu_exec_nocache(CPUState *cpu, int max_cycles,
 203                              TranslationBlock *orig_tb, bool ignore_icount)
 204 {
 205     TranslationBlock *tb;
 206     bool old_tb_flushed;
 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     old_tb_flushed = cpu->tb_flushed;
 214     cpu->tb_flushed = false;
 215     tb = tb_gen_code(cpu, orig_tb->pc, orig_tb->cs_base, orig_tb->flags,
 216                      max_cycles | CF_NOCACHE
 217                          | (ignore_icount ? CF_IGNORE_ICOUNT : 0));
 218     tb->orig_tb = cpu->tb_flushed ? NULL : orig_tb;
 219     cpu->tb_flushed |= old_tb_flushed;
 220     /* execute the generated code */
 221     trace_exec_tb_nocache(tb, tb->pc);
 222     cpu_tb_exec(cpu, tb);
 223     tb_phys_invalidate(tb, -1);
 224     tb_free(tb);
 225 }
 226 #endif
 227
 228 struct tb_desc {
 229     target_ulong pc;
 230     target_ulong cs_base;
 231     CPUArchState *env;
 232     tb_page_addr_t phys_page1;
 233     uint32_t flags;
 234 };
 235
 236 static bool tb_cmp(const void *p, const void *d)
 237 {
 238     const TranslationBlock *tb = p;
 239     const struct tb_desc *desc = d;
 240
 241     if (tb->pc == desc->pc &&
 242         tb->page_addr[0] == desc->phys_page1 &&
 243         tb->cs_base == desc->cs_base &&
 244         tb->flags == desc->flags &&
 245         !atomic_read(&tb->invalid)) {
 246         /* check next page if needed */
 247         if (tb->page_addr[1] == -1) {
 248             return true;
 249         } else {
 250             tb_page_addr_t phys_page2;
 251             target_ulong virt_page2;
 252
 253             virt_page2 = (desc->pc & TARGET_PAGE_MASK) + TARGET_PAGE_SIZE;
 254             phys_page2 = get_page_addr_code(desc->env, virt_page2);
 255             if (tb->page_addr[1] == phys_page2) {
 256                 return true;
 257             }
 258         }
 259     }
 260     return false;
 261 }
 262
 263 static TranslationBlock *tb_htable_lookup(CPUState *cpu,
 264                                           target_ulong pc,
 265                                           target_ulong cs_base,
 266                                           uint32_t flags)
 267 {
 268     tb_page_addr_t phys_pc;
 269     struct tb_desc desc;
 270     uint32_t h;
 271
 272     desc.env = (CPUArchState *)cpu->env_ptr;
 273     desc.cs_base = cs_base;
 274     desc.flags = flags;
 275     desc.pc = pc;
 276     phys_pc = get_page_addr_code(desc.env, pc);
 277     desc.phys_page1 = phys_pc & TARGET_PAGE_MASK;
 278     h = tb_hash_func(phys_pc, pc, flags);
 279     return qht_lookup(&tcg_ctx.tb_ctx.htable, tb_cmp, &desc, h);
 280 }
 281
 282 static inline TranslationBlock *tb_find(CPUState *cpu,
 283                                         TranslationBlock *last_tb,
 284                                         int tb_exit)
 285 {
 286     CPUArchState *env = (CPUArchState *)cpu->env_ptr;
 287     TranslationBlock *tb;
 288     target_ulong cs_base, pc;
 289     uint32_t flags;
 290     bool have_tb_lock = false;
 291
 292     /* we record a subset of the CPU state. It will
 293        always be the same before a given translated block
 294        is executed. */
 295     cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
 296     tb = atomic_rcu_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)]);
 297     if (unlikely(!tb || tb->pc != pc || tb->cs_base != cs_base ||
 298                  tb->flags != flags)) {
 299         tb = tb_htable_lookup(cpu, pc, cs_base, flags);
 300         if (!tb) {
 301
 302             /* mmap_lock is needed by tb_gen_code, and mmap_lock must be
 303              * taken outside tb_lock. As system emulation is currently
 304              * single threaded the locks are NOPs.
 305              */
 306             mmap_lock();
 307             tb_lock();
 308             have_tb_lock = true;
 309
 310             /* There's a chance that our desired tb has been translated while
 311              * taking the locks so we check again inside the lock.
 312              */
 313             tb = tb_htable_lookup(cpu, pc, cs_base, flags);
 314             if (!tb) {
 315                 /* if no translated code available, then translate it now */
 316                 tb = tb_gen_code(cpu, pc, cs_base, flags, 0);
 317             }
 318
 319             mmap_unlock();
 320         }
 321
 322         /* We add the TB in the virtual pc hash table for the fast lookup */
 323         atomic_set(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)], tb);
 324     }
 325 #ifndef CONFIG_USER_ONLY
 326     /* We don't take care of direct jumps when address mapping changes in
 327      * system emulation. So it's not safe to make a direct jump to a TB
 328      * spanning two pages because the mapping for the second page can change.
 329      */
 330     if (tb->page_addr[1] != -1) {
 331         last_tb = NULL;
 332     }
 333 #endif
 334     /* See if we can patch the calling TB. */
 335     if (last_tb && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
 336         if (!have_tb_lock) {
 337             tb_lock();
 338             have_tb_lock = true;
 339         }
 340         /* Check if translation buffer has been flushed */
 341         if (cpu->tb_flushed) {
 342             cpu->tb_flushed = false;
 343         } else if (!tb->invalid) {
 344             tb_add_jump(last_tb, tb_exit, tb);
 345         }
 346     }
 347     if (have_tb_lock) {
 348         tb_unlock();
 349     }
 350     return tb;
 351 }
 352
 353 static inline bool cpu_handle_halt(CPUState *cpu)
 354 {
 355     if (cpu->halted) {
 356 #if defined(TARGET_I386) && !defined(CONFIG_USER_ONLY)
 357         if ((cpu->interrupt_request & CPU_INTERRUPT_POLL)
 358             && replay_interrupt()) {
 359             X86CPU *x86_cpu = X86_CPU(cpu);
 360             apic_poll_irq(x86_cpu->apic_state);
 361             cpu_reset_interrupt(cpu, CPU_INTERRUPT_POLL);
 362         }
 363 #endif
 364         if (!cpu_has_work(cpu)) {
 365             current_cpu = NULL;
 366             return true;
 367         }
 368
 369         cpu->halted = 0;
 370     }
 371
 372     return false;
 373 }
 374
 375 static inline void cpu_handle_debug_exception(CPUState *cpu)
 376 {
 377     CPUClass *cc = CPU_GET_CLASS(cpu);
 378     CPUWatchpoint *wp;
 379
 380     if (!cpu->watchpoint_hit) {
 381         QTAILQ_FOREACH(wp, &cpu->watchpoints, entry) {
 382             wp->flags &= ~BP_WATCHPOINT_HIT;
 383         }
 384     }
 385
 386     cc->debug_excp_handler(cpu);
 387 }
 388
 389 static inline bool cpu_handle_exception(CPUState *cpu, int *ret)
 390 {
 391     if (cpu->exception_index >= 0) {
 392         if (cpu->exception_index >= EXCP_INTERRUPT) {
 393             /* exit request from the cpu execution loop */
 394             *ret = cpu->exception_index;
 395             if (*ret == EXCP_DEBUG) {
 396                 cpu_handle_debug_exception(cpu);
 397             }
 398             cpu->exception_index = -1;
 399             return true;
 400         } else {
 401 #if defined(CONFIG_USER_ONLY)
 402             /* if user mode only, we simulate a fake exception
 403                which will be handled outside the cpu execution
 404                loop */
 405 #if defined(TARGET_I386)
 406             CPUClass *cc = CPU_GET_CLASS(cpu);
 407             cc->do_interrupt(cpu);
 408 #endif
 409             *ret = cpu->exception_index;
 410             cpu->exception_index = -1;
 411             return true;
 412 #else
 413             if (replay_exception()) {
 414                 CPUClass *cc = CPU_GET_CLASS(cpu);
 415                 cc->do_interrupt(cpu);
 416                 cpu->exception_index = -1;
 417             } else if (!replay_has_interrupt()) {
 418                 /* give a chance to iothread in replay mode */
 419                 *ret = EXCP_INTERRUPT;
 420                 return true;
 421             }
 422 #endif
 423         }
 424 #ifndef CONFIG_USER_ONLY
 425     } else if (replay_has_exception()
 426                && cpu->icount_decr.u16.low + cpu->icount_extra == 0) {
 427         /* try to cause an exception pending in the log */
 428         cpu_exec_nocache(cpu, 1, tb_find(cpu, NULL, 0), true);
 429         *ret = -1;
 430         return true;
 431 #endif
 432     }
 433
 434     return false;
 435 }
 436
 437 static inline void cpu_handle_interrupt(CPUState *cpu,
 438                                         TranslationBlock **last_tb)
 439 {
 440     CPUClass *cc = CPU_GET_CLASS(cpu);
 441     int interrupt_request = cpu->interrupt_request;
 442
 443     if (unlikely(interrupt_request)) {
 444         if (unlikely(cpu->singlestep_enabled & SSTEP_NOIRQ)) {
 445             /* Mask out external interrupts for this step. */
 446             interrupt_request &= ~CPU_INTERRUPT_SSTEP_MASK;
 447         }
 448         if (interrupt_request & CPU_INTERRUPT_DEBUG) {
 449             cpu->interrupt_request &= ~CPU_INTERRUPT_DEBUG;
 450             cpu->exception_index = EXCP_DEBUG;
 451             cpu_loop_exit(cpu);
 452         }
 453         if (replay_mode == REPLAY_MODE_PLAY && !replay_has_interrupt()) {
 454             /* Do nothing */
 455         } else if (interrupt_request & CPU_INTERRUPT_HALT) {
 456             replay_interrupt();
 457             cpu->interrupt_request &= ~CPU_INTERRUPT_HALT;
 458             cpu->halted = 1;
 459             cpu->exception_index = EXCP_HLT;
 460             cpu_loop_exit(cpu);
 461         }
 462 #if defined(TARGET_I386)
 463         else if (interrupt_request & CPU_INTERRUPT_INIT) {
 464             X86CPU *x86_cpu = X86_CPU(cpu);
 465             CPUArchState *env = &x86_cpu->env;
 466             replay_interrupt();
 467             cpu_svm_check_intercept_param(env, SVM_EXIT_INIT, 0);
 468             do_cpu_init(x86_cpu);
 469             cpu->exception_index = EXCP_HALTED;
 470             cpu_loop_exit(cpu);
 471         }
 472 #else
 473         else if (interrupt_request & CPU_INTERRUPT_RESET) {
 474             replay_interrupt();
 475             cpu_reset(cpu);
 476             cpu_loop_exit(cpu);
 477         }
 478 #endif
 479         /* The target hook has 3 exit conditions:
 480            False when the interrupt isn't processed,
 481            True when it is, and we should restart on a new TB,
 482            and via longjmp via cpu_loop_exit.  */
 483         else {
 484             replay_interrupt();
 485             if (cc->cpu_exec_interrupt(cpu, interrupt_request)) {
 486                 *last_tb = NULL;
 487             }
 488             /* The target hook may have updated the 'cpu->interrupt_request';
 489              * reload the 'interrupt_request' value */
 490             interrupt_request = cpu->interrupt_request;
 491         }
 492         if (interrupt_request & CPU_INTERRUPT_EXITTB) {
 493             cpu->interrupt_request &= ~CPU_INTERRUPT_EXITTB;
 494             /* ensure that no TB jump will be modified as
 495                the program flow was changed */
 496             *last_tb = NULL;
 497         }
 498     }
 499     if (unlikely(cpu->exit_request || replay_has_interrupt())) {
 500         cpu->exit_request = 0;
 501         cpu->exception_index = EXCP_INTERRUPT;
 502         cpu_loop_exit(cpu);
 503     }
 504 }
 505
 506 static inline void cpu_loop_exec_tb(CPUState *cpu, TranslationBlock *tb,
 507                                     TranslationBlock **last_tb, int *tb_exit,
 508                                     SyncClocks *sc)
 509 {
 510     uintptr_t ret;
 511
 512     if (unlikely(cpu->exit_request)) {
 513         return;
 514     }
 515
 516     trace_exec_tb(tb, tb->pc);
 517     ret = cpu_tb_exec(cpu, tb);
 518     *last_tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
 519     *tb_exit = ret & TB_EXIT_MASK;
 520     switch (*tb_exit) {
 521     case TB_EXIT_REQUESTED:
 522         /* Something asked us to stop executing
 523          * chained TBs; just continue round the main
 524          * loop. Whatever requested the exit will also
 525          * have set something else (eg exit_request or
 526          * interrupt_request) which we will handle
 527          * next time around the loop.  But we need to
 528          * ensure the tcg_exit_req read in generated code
 529          * comes before the next read of cpu->exit_request
 530          * or cpu->interrupt_request.
 531          */
 532         smp_rmb();
 533         *last_tb = NULL;
 534         break;
 535     case TB_EXIT_ICOUNT_EXPIRED:
 536     {
 537         /* Instruction counter expired.  */
 538 #ifdef CONFIG_USER_ONLY
 539         abort();
 540 #else
 541         int insns_left = cpu->icount_decr.u32;
 542         if (cpu->icount_extra && insns_left >= 0) {
 543             /* Refill decrementer and continue execution.  */
 544             cpu->icount_extra += insns_left;
 545             insns_left = MIN(0xffff, cpu->icount_extra);
 546             cpu->icount_extra -= insns_left;
 547             cpu->icount_decr.u16.low = insns_left;
 548         } else {
 549             if (insns_left > 0) {
 550                 /* Execute remaining instructions.  */
 551                 cpu_exec_nocache(cpu, insns_left, *last_tb, false);
 552                 align_clocks(sc, cpu);
 553             }
 554             cpu->exception_index = EXCP_INTERRUPT;
 555             *last_tb = NULL;
 556             cpu_loop_exit(cpu);
 557         }
 558         break;
 559 #endif
 560     }
 561     default:
 562         break;
 563     }
 564 }
 565
 566 /* main execution loop */
 567
 568 int cpu_exec(CPUState *cpu)
 569 {
 570     CPUClass *cc = CPU_GET_CLASS(cpu);
 571     int ret;
 572     SyncClocks sc;
 573
 574     /* replay_interrupt may need current_cpu */
 575     current_cpu = cpu;
 576
 577     if (cpu_handle_halt(cpu)) {
 578         return EXCP_HALTED;
 579     }
 580
 581     atomic_mb_set(&tcg_current_cpu, cpu);
 582     rcu_read_lock();
 583
 584     if (unlikely(atomic_mb_read(&exit_request))) {
 585         cpu->exit_request = 1;
 586     }
 587
 588     cc->cpu_exec_enter(cpu);
 589
 590     /* Calculate difference between guest clock and host clock.
 591      * This delay includes the delay of the last cycle, so
 592      * what we have to do is sleep until it is 0. As for the
 593      * advance/delay we gain here, we try to fix it next time.
 594      */
 595     init_delay_params(&sc, cpu);
 596
 597     for(;;) {
 598         /* prepare setjmp context for exception handling */
 599         if (sigsetjmp(cpu->jmp_env, 0) == 0) {
 600             TranslationBlock *tb, *last_tb = NULL;
 601             int tb_exit = 0;
 602
 603             /* if an exception is pending, we execute it here */
 604             if (cpu_handle_exception(cpu, &ret)) {
 605                 break;
 606             }
 607
 608             atomic_mb_set(&cpu->tb_flushed, false); /* reset before first TB lookup */
 609             for(;;) {
 610                 cpu_handle_interrupt(cpu, &last_tb);
 611                 tb = tb_find(cpu, last_tb, tb_exit);
 612                 cpu_loop_exec_tb(cpu, tb, &last_tb, &tb_exit, &sc);
 613                 /* Try to align the host and virtual clocks
 614                    if the guest is in advance */
 615                 align_clocks(&sc, cpu);
 616             } /* for(;;) */
 617         } else {
 618 #if defined(__clang__) || !QEMU_GNUC_PREREQ(4, 6)
 619             /* Some compilers wrongly smash all local variables after
 620              * siglongjmp. There were bug reports for gcc 4.5.0 and clang.
 621              * Reload essential local variables here for those compilers.
 622              * Newer versions of gcc would complain about this code (-Wclobbered). */
 623             cpu = current_cpu;
 624             cc = CPU_GET_CLASS(cpu);
 625 #else /* buggy compiler */
 626             /* Assert that the compiler does not smash local variables. */
 627             g_assert(cpu == current_cpu);
 628             g_assert(cc == CPU_GET_CLASS(cpu));
 629 #endif /* buggy compiler */
 630             cpu->can_do_io = 1;
 631             tb_lock_reset();
 632         }
 633     } /* for(;;) */
 634
 635     cc->cpu_exec_exit(cpu);
 636     rcu_read_unlock();
 637
 638     /* fail safe : never use current_cpu outside cpu_exec() */
 639     current_cpu = NULL;
 640
 641     /* Does not need atomic_mb_set because a spurious wakeup is okay.  */
 642     atomic_set(&tcg_current_cpu, NULL);
 643     return ret;
 644 }