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 "tcg.h"
  24 #include "qemu/atomic.h"
  25 #include "sysemu/qtest.h"
  26 #include "qemu/timer.h"
  27 #include "exec/address-spaces.h"
  28 #include "qemu/rcu.h"
  29 #include "exec/tb-hash.h"
  30 #include "exec/log.h"
  31 #if defined(TARGET_I386) && !defined(CONFIG_USER_ONLY)
  32 #include "hw/i386/apic.h"
  33 #endif
  34 #include "sysemu/replay.h"
  35
  36 /* -icount align implementation. */
  37
  38 typedef struct SyncClocks {
  39     int64_t diff_clk;
  40     int64_t last_cpu_icount;
  41     int64_t realtime_clock;
  42 } SyncClocks;
  43
  44 #if !defined(CONFIG_USER_ONLY)
  45 /* Allow the guest to have a max 3ms advance.
  46  * The difference between the 2 clocks could therefore
  47  * oscillate around 0.
  48  */
  49 #define VM_CLOCK_ADVANCE 3000000
  50 #define THRESHOLD_REDUCE 1.5
  51 #define MAX_DELAY_PRINT_RATE 2000000000LL
  52 #define MAX_NB_PRINTS 100
  53
  54 static void align_clocks(SyncClocks *sc, const CPUState *cpu)
  55 {
  56     int64_t cpu_icount;
  57
  58     if (!icount_align_option) {
  59         return;
  60     }
  61
  62     cpu_icount = cpu->icount_extra + cpu->icount_decr.u16.low;
  63     sc->diff_clk += cpu_icount_to_ns(sc->last_cpu_icount - cpu_icount);
  64     sc->last_cpu_icount = cpu_icount;
  65
  66     if (sc->diff_clk > VM_CLOCK_ADVANCE) {
  67 #ifndef _WIN32
  68         struct timespec sleep_delay, rem_delay;
  69         sleep_delay.tv_sec = sc->diff_clk / 1000000000LL;
  70         sleep_delay.tv_nsec = sc->diff_clk % 1000000000LL;
  71         if (nanosleep(&sleep_delay, &rem_delay) < 0) {
  72             sc->diff_clk = rem_delay.tv_sec * 1000000000LL + rem_delay.tv_nsec;
  73         } else {
  74             sc->diff_clk = 0;
  75         }
  76 #else
  77         Sleep(sc->diff_clk / SCALE_MS);
  78         sc->diff_clk = 0;
  79 #endif
  80     }
  81 }
  82
  83 static void print_delay(const SyncClocks *sc)
  84 {
  85     static float threshold_delay;
  86     static int64_t last_realtime_clock;
  87     static int nb_prints;
  88
  89     if (icount_align_option &&
  90         sc->realtime_clock - last_realtime_clock >= MAX_DELAY_PRINT_RATE &&
  91         nb_prints < MAX_NB_PRINTS) {
  92         if ((-sc->diff_clk / (float)1000000000LL > threshold_delay) ||
  93             (-sc->diff_clk / (float)1000000000LL <
  94              (threshold_delay - THRESHOLD_REDUCE))) {
  95             threshold_delay = (-sc->diff_clk / 1000000000LL) + 1;
  96             printf("Warning: The guest is now late by %.1f to %.1f seconds\n",
  97                    threshold_delay - 1,
  98                    threshold_delay);
  99             nb_prints++;
 100             last_realtime_clock = sc->realtime_clock;
 101         }
 102     }
 103 }
 104
 105 static void init_delay_params(SyncClocks *sc,
 106                               const CPUState *cpu)
 107 {
 108     if (!icount_align_option) {
 109         return;
 110     }
 111     sc->realtime_clock = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL_RT);
 112     sc->diff_clk = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) - sc->realtime_clock;
 113     sc->last_cpu_icount = cpu->icount_extra + cpu->icount_decr.u16.low;
 114     if (sc->diff_clk < max_delay) {
 115         max_delay = sc->diff_clk;
 116     }
 117     if (sc->diff_clk > max_advance) {
 118         max_advance = sc->diff_clk;
 119     }
 120
 121     /* Print every 2s max if the guest is late. We limit the number
 122        of printed messages to NB_PRINT_MAX(currently 100) */
 123     print_delay(sc);
 124 }
 125 #else
 126 static void align_clocks(SyncClocks *sc, const CPUState *cpu)
 127 {
 128 }
 129
 130 static void init_delay_params(SyncClocks *sc, const CPUState *cpu)
 131 {
 132 }
 133 #endif /* CONFIG USER ONLY */
 134
 135 /* Execute a TB, and fix up the CPU state afterwards if necessary */
 136 static inline tcg_target_ulong cpu_tb_exec(CPUState *cpu, TranslationBlock *itb)
 137 {
 138     CPUArchState *env = cpu->env_ptr;
 139     uintptr_t ret;
 140     TranslationBlock *last_tb;
 141     int tb_exit;
 142     uint8_t *tb_ptr = itb->tc_ptr;
 143
 144     qemu_log_mask_and_addr(CPU_LOG_EXEC, itb->pc,
 145                            "Trace %p [" TARGET_FMT_lx "] %s\n",
 146                            itb->tc_ptr, itb->pc, lookup_symbol(itb->pc));
 147
 148 #if defined(DEBUG_DISAS)
 149     if (qemu_loglevel_mask(CPU_LOG_TB_CPU)) {
 150 #if defined(TARGET_I386)
 151         log_cpu_state(cpu, CPU_DUMP_CCOP);
 152 #elif defined(TARGET_M68K)
 153         /* ??? Should not modify env state for dumping.  */
 154         cpu_m68k_flush_flags(env, env->cc_op);
 155         env->cc_op = CC_OP_FLAGS;
 156         env->sr = (env->sr & 0xffe0) | env->cc_dest | (env->cc_x << 4);
 157         log_cpu_state(cpu, 0);
 158 #else
 159         log_cpu_state(cpu, 0);
 160 #endif
 161     }
 162 #endif /* DEBUG_DISAS */
 163
 164     cpu->can_do_io = !use_icount;
 165     ret = tcg_qemu_tb_exec(env, tb_ptr);
 166     cpu->can_do_io = 1;
 167     last_tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
 168     tb_exit = ret & TB_EXIT_MASK;
 169     trace_exec_tb_exit(last_tb, tb_exit);
 170
 171     if (tb_exit > TB_EXIT_IDX1) {
 172         /* We didn't start executing this TB (eg because the instruction
 173          * counter hit zero); we must restore the guest PC to the address
 174          * of the start of the TB.
 175          */
 176         CPUClass *cc = CPU_GET_CLASS(cpu);
 177         qemu_log_mask_and_addr(CPU_LOG_EXEC, last_tb->pc,
 178                                "Stopped execution of TB chain before %p ["
 179                                TARGET_FMT_lx "] %s\n",
 180                                last_tb->tc_ptr, last_tb->pc,
 181                                lookup_symbol(last_tb->pc));
 182         if (cc->synchronize_from_tb) {
 183             cc->synchronize_from_tb(cpu, last_tb);
 184         } else {
 185             assert(cc->set_pc);
 186             cc->set_pc(cpu, last_tb->pc);
 187         }
 188     }
 189     if (tb_exit == TB_EXIT_REQUESTED) {
 190         /* We were asked to stop executing TBs (probably a pending
 191          * interrupt. We've now stopped, so clear the flag.
 192          */
 193         cpu->tcg_exit_req = 0;
 194     }
 195     return ret;
 196 }
 197
 198 #ifndef CONFIG_USER_ONLY
 199 /* Execute the code without caching the generated code. An interpreter
 200    could be used if available. */
 201 static void cpu_exec_nocache(CPUState *cpu, int max_cycles,
 202                              TranslationBlock *orig_tb, bool ignore_icount)
 203 {
 204     TranslationBlock *tb;
 205     bool old_tb_flushed;
 206
 207     /* Should never happen.
 208        We only end up here when an existing TB is too long.  */
 209     if (max_cycles > CF_COUNT_MASK)
 210         max_cycles = CF_COUNT_MASK;
 211
 212     old_tb_flushed = cpu->tb_flushed;
 213     cpu->tb_flushed = false;
 214     tb = tb_gen_code(cpu, orig_tb->pc, orig_tb->cs_base, orig_tb->flags,
 215                      max_cycles | CF_NOCACHE
 216                          | (ignore_icount ? CF_IGNORE_ICOUNT : 0));
 217     tb->orig_tb = cpu->tb_flushed ? NULL : orig_tb;
 218     cpu->tb_flushed |= old_tb_flushed;
 219     /* execute the generated code */
 220     trace_exec_tb_nocache(tb, tb->pc);
 221     cpu_tb_exec(cpu, tb);
 222     tb_phys_invalidate(tb, -1);
 223     tb_free(tb);
 224 }
 225 #endif
 226
 227 static TranslationBlock *tb_find_physical(CPUState *cpu,
 228                                           target_ulong pc,
 229                                           target_ulong cs_base,
 230                                           uint32_t flags)
 231 {
 232     CPUArchState *env = (CPUArchState *)cpu->env_ptr;
 233     TranslationBlock *tb, **tb_hash_head, **ptb1;
 234     unsigned int h;
 235     tb_page_addr_t phys_pc, phys_page1;
 236
 237     /* find translated block using physical mappings */
 238     phys_pc = get_page_addr_code(env, pc);
 239     phys_page1 = phys_pc & TARGET_PAGE_MASK;
 240     h = tb_phys_hash_func(phys_pc);
 241
 242     /* Start at head of the hash entry */
 243     ptb1 = tb_hash_head = &tcg_ctx.tb_ctx.tb_phys_hash[h];
 244     tb = *ptb1;
 245
 246     while (tb) {
 247         if (tb->pc == pc &&
 248             tb->page_addr[0] == phys_page1 &&
 249             tb->cs_base == cs_base &&
 250             tb->flags == flags) {
 251
 252             if (tb->page_addr[1] == -1) {
 253                 /* done, we have a match */
 254                 break;
 255             } else {
 256                 /* check next page if needed */
 257                 target_ulong virt_page2 = (pc & TARGET_PAGE_MASK) +
 258                                           TARGET_PAGE_SIZE;
 259                 tb_page_addr_t phys_page2 = get_page_addr_code(env, virt_page2);
 260
 261                 if (tb->page_addr[1] == phys_page2) {
 262                     break;
 263                 }
 264             }
 265         }
 266
 267         ptb1 = &tb->phys_hash_next;
 268         tb = *ptb1;
 269     }
 270
 271     if (tb) {
 272         /* Move the TB to the head of the list */
 273         *ptb1 = tb->phys_hash_next;
 274         tb->phys_hash_next = *tb_hash_head;
 275         *tb_hash_head = tb;
 276     }
 277     return tb;
 278 }
 279
 280 static TranslationBlock *tb_find_slow(CPUState *cpu,
 281                                       target_ulong pc,
 282                                       target_ulong cs_base,
 283                                       uint32_t flags)
 284 {
 285     TranslationBlock *tb;
 286
 287     tb = tb_find_physical(cpu, pc, cs_base, flags);
 288     if (tb) {
 289         goto found;
 290     }
 291
 292 #ifdef CONFIG_USER_ONLY
 293     /* mmap_lock is needed by tb_gen_code, and mmap_lock must be
 294      * taken outside tb_lock.  Since we're momentarily dropping
 295      * tb_lock, there's a chance that our desired tb has been
 296      * translated.
 297      */
 298     tb_unlock();
 299     mmap_lock();
 300     tb_lock();
 301     tb = tb_find_physical(cpu, pc, cs_base, flags);
 302     if (tb) {
 303         mmap_unlock();
 304         goto found;
 305     }
 306 #endif
 307
 308     /* if no translated code available, then translate it now */
 309     tb = tb_gen_code(cpu, pc, cs_base, flags, 0);
 310
 311 #ifdef CONFIG_USER_ONLY
 312     mmap_unlock();
 313 #endif
 314
 315 found:
 316     /* we add the TB in the virtual pc hash table */
 317     cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)] = tb;
 318     return tb;
 319 }
 320
 321 static inline TranslationBlock *tb_find_fast(CPUState *cpu,
 322                                              TranslationBlock **last_tb,
 323                                              int tb_exit)
 324 {
 325     CPUArchState *env = (CPUArchState *)cpu->env_ptr;
 326     TranslationBlock *tb;
 327     target_ulong cs_base, pc;
 328     uint32_t flags;
 329
 330     /* we record a subset of the CPU state. It will
 331        always be the same before a given translated block
 332        is executed. */
 333     cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
 334     tb_lock();
 335     tb = cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)];
 336     if (unlikely(!tb || tb->pc != pc || tb->cs_base != cs_base ||
 337                  tb->flags != flags)) {
 338         tb = tb_find_slow(cpu, pc, cs_base, flags);
 339     }
 340     if (cpu->tb_flushed) {
 341         /* Ensure that no TB jump will be modified as the
 342          * translation buffer has been flushed.
 343          */
 344         *last_tb = NULL;
 345         cpu->tb_flushed = false;
 346     }
 347     /* See if we can patch the calling TB. */
 348     if (*last_tb && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
 349         tb_add_jump(*last_tb, tb_exit, tb);
 350     }
 351     tb_unlock();
 352     return tb;
 353 }
 354
 355 static inline bool cpu_handle_halt(CPUState *cpu)
 356 {
 357     if (cpu->halted) {
 358 #if defined(TARGET_I386) && !defined(CONFIG_USER_ONLY)
 359         if ((cpu->interrupt_request & CPU_INTERRUPT_POLL)
 360             && replay_interrupt()) {
 361             X86CPU *x86_cpu = X86_CPU(cpu);
 362             apic_poll_irq(x86_cpu->apic_state);
 363             cpu_reset_interrupt(cpu, CPU_INTERRUPT_POLL);
 364         }
 365 #endif
 366         if (!cpu_has_work(cpu)) {
 367             current_cpu = NULL;
 368             return true;
 369         }
 370
 371         cpu->halted = 0;
 372     }
 373
 374     return false;
 375 }
 376
 377 static inline void cpu_handle_debug_exception(CPUState *cpu)
 378 {
 379     CPUClass *cc = CPU_GET_CLASS(cpu);
 380     CPUWatchpoint *wp;
 381
 382     if (!cpu->watchpoint_hit) {
 383         QTAILQ_FOREACH(wp, &cpu->watchpoints, entry) {
 384             wp->flags &= ~BP_WATCHPOINT_HIT;
 385         }
 386     }
 387
 388     cc->debug_excp_handler(cpu);
 389 }
 390
 391 static inline bool cpu_handle_exception(CPUState *cpu, int *ret)
 392 {
 393     if (cpu->exception_index >= 0) {
 394         if (cpu->exception_index >= EXCP_INTERRUPT) {
 395             /* exit request from the cpu execution loop */
 396             *ret = cpu->exception_index;
 397             if (*ret == EXCP_DEBUG) {
 398                 cpu_handle_debug_exception(cpu);
 399             }
 400             cpu->exception_index = -1;
 401             return true;
 402         } else {
 403 #if defined(CONFIG_USER_ONLY)
 404             /* if user mode only, we simulate a fake exception
 405                which will be handled outside the cpu execution
 406                loop */
 407 #if defined(TARGET_I386)
 408             CPUClass *cc = CPU_GET_CLASS(cpu);
 409             cc->do_interrupt(cpu);
 410 #endif
 411             *ret = cpu->exception_index;
 412             cpu->exception_index = -1;
 413             return true;
 414 #else
 415             if (replay_exception()) {
 416                 CPUClass *cc = CPU_GET_CLASS(cpu);
 417                 cc->do_interrupt(cpu);
 418                 cpu->exception_index = -1;
 419             } else if (!replay_has_interrupt()) {
 420                 /* give a chance to iothread in replay mode */
 421                 *ret = EXCP_INTERRUPT;
 422                 return true;
 423             }
 424 #endif
 425         }
 426 #ifndef CONFIG_USER_ONLY
 427     } else if (replay_has_exception()
 428                && cpu->icount_decr.u16.low + cpu->icount_extra == 0) {
 429         /* try to cause an exception pending in the log */
 430         TranslationBlock *last_tb = NULL; /* Avoid chaining TBs */
 431         cpu_exec_nocache(cpu, 1, tb_find_fast(cpu, &last_tb, 0), true);
 432         *ret = -1;
 433         return true;
 434 #endif
 435     }
 436
 437     return false;
 438 }
 439
 440 static inline void cpu_handle_interrupt(CPUState *cpu,
 441                                         TranslationBlock **last_tb)
 442 {
 443     CPUClass *cc = CPU_GET_CLASS(cpu);
 444     int interrupt_request = cpu->interrupt_request;
 445
 446     if (unlikely(interrupt_request)) {
 447         if (unlikely(cpu->singlestep_enabled & SSTEP_NOIRQ)) {
 448             /* Mask out external interrupts for this step. */
 449             interrupt_request &= ~CPU_INTERRUPT_SSTEP_MASK;
 450         }
 451         if (interrupt_request & CPU_INTERRUPT_DEBUG) {
 452             cpu->interrupt_request &= ~CPU_INTERRUPT_DEBUG;
 453             cpu->exception_index = EXCP_DEBUG;
 454             cpu_loop_exit(cpu);
 455         }
 456         if (replay_mode == REPLAY_MODE_PLAY && !replay_has_interrupt()) {
 457             /* Do nothing */
 458         } else if (interrupt_request & CPU_INTERRUPT_HALT) {
 459             replay_interrupt();
 460             cpu->interrupt_request &= ~CPU_INTERRUPT_HALT;
 461             cpu->halted = 1;
 462             cpu->exception_index = EXCP_HLT;
 463             cpu_loop_exit(cpu);
 464         }
 465 #if defined(TARGET_I386)
 466         else if (interrupt_request & CPU_INTERRUPT_INIT) {
 467             X86CPU *x86_cpu = X86_CPU(cpu);
 468             CPUArchState *env = &x86_cpu->env;
 469             replay_interrupt();
 470             cpu_svm_check_intercept_param(env, SVM_EXIT_INIT, 0);
 471             do_cpu_init(x86_cpu);
 472             cpu->exception_index = EXCP_HALTED;
 473             cpu_loop_exit(cpu);
 474         }
 475 #else
 476         else if (interrupt_request & CPU_INTERRUPT_RESET) {
 477             replay_interrupt();
 478             cpu_reset(cpu);
 479             cpu_loop_exit(cpu);
 480         }
 481 #endif
 482         /* The target hook has 3 exit conditions:
 483            False when the interrupt isn't processed,
 484            True when it is, and we should restart on a new TB,
 485            and via longjmp via cpu_loop_exit.  */
 486         else {
 487             replay_interrupt();
 488             if (cc->cpu_exec_interrupt(cpu, interrupt_request)) {
 489                 *last_tb = NULL;
 490             }
 491             /* The target hook may have updated the 'cpu->interrupt_request';
 492              * reload the 'interrupt_request' value */
 493             interrupt_request = cpu->interrupt_request;
 494         }
 495         if (interrupt_request & CPU_INTERRUPT_EXITTB) {
 496             cpu->interrupt_request &= ~CPU_INTERRUPT_EXITTB;
 497             /* ensure that no TB jump will be modified as
 498                the program flow was changed */
 499             *last_tb = NULL;
 500         }
 501     }
 502     if (unlikely(cpu->exit_request || replay_has_interrupt())) {
 503         cpu->exit_request = 0;
 504         cpu->exception_index = EXCP_INTERRUPT;
 505         cpu_loop_exit(cpu);
 506     }
 507 }
 508
 509 static inline void cpu_loop_exec_tb(CPUState *cpu, TranslationBlock *tb,
 510                                     TranslationBlock **last_tb, int *tb_exit,
 511                                     SyncClocks *sc)
 512 {
 513     uintptr_t ret;
 514
 515     if (unlikely(cpu->exit_request)) {
 516         return;
 517     }
 518
 519     trace_exec_tb(tb, tb->pc);
 520     ret = cpu_tb_exec(cpu, tb);
 521     *last_tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
 522     *tb_exit = ret & TB_EXIT_MASK;
 523     switch (*tb_exit) {
 524     case TB_EXIT_REQUESTED:
 525         /* Something asked us to stop executing
 526          * chained TBs; just continue round the main
 527          * loop. Whatever requested the exit will also
 528          * have set something else (eg exit_request or
 529          * interrupt_request) which we will handle
 530          * next time around the loop.  But we need to
 531          * ensure the tcg_exit_req read in generated code
 532          * comes before the next read of cpu->exit_request
 533          * or cpu->interrupt_request.
 534          */
 535         smp_rmb();
 536         *last_tb = NULL;
 537         break;
 538     case TB_EXIT_ICOUNT_EXPIRED:
 539     {
 540         /* Instruction counter expired.  */
 541 #ifdef CONFIG_USER_ONLY
 542         abort();
 543 #else
 544         int insns_left = cpu->icount_decr.u32;
 545         if (cpu->icount_extra && insns_left >= 0) {
 546             /* Refill decrementer and continue execution.  */
 547             cpu->icount_extra += insns_left;
 548             insns_left = MIN(0xffff, cpu->icount_extra);
 549             cpu->icount_extra -= insns_left;
 550             cpu->icount_decr.u16.low = insns_left;
 551         } else {
 552             if (insns_left > 0) {
 553                 /* Execute remaining instructions.  */
 554                 cpu_exec_nocache(cpu, insns_left, *last_tb, false);
 555                 align_clocks(sc, cpu);
 556             }
 557             cpu->exception_index = EXCP_INTERRUPT;
 558             *last_tb = NULL;
 559             cpu_loop_exit(cpu);
 560         }
 561         break;
 562 #endif
 563     }
 564     default:
 565         break;
 566     }
 567 }
 568
 569 /* main execution loop */
 570
 571 int cpu_exec(CPUState *cpu)
 572 {
 573     CPUClass *cc = CPU_GET_CLASS(cpu);
 574     int ret;
 575     SyncClocks sc;
 576
 577     /* replay_interrupt may need current_cpu */
 578     current_cpu = cpu;
 579
 580     if (cpu_handle_halt(cpu)) {
 581         return EXCP_HALTED;
 582     }
 583
 584     atomic_mb_set(&tcg_current_cpu, cpu);
 585     rcu_read_lock();
 586
 587     if (unlikely(atomic_mb_read(&exit_request))) {
 588         cpu->exit_request = 1;
 589     }
 590
 591     cc->cpu_exec_enter(cpu);
 592
 593     /* Calculate difference between guest clock and host clock.
 594      * This delay includes the delay of the last cycle, so
 595      * what we have to do is sleep until it is 0. As for the
 596      * advance/delay we gain here, we try to fix it next time.
 597      */
 598     init_delay_params(&sc, cpu);
 599
 600     for(;;) {
 601         TranslationBlock *tb, *last_tb;
 602         int tb_exit = 0;
 603
 604         /* prepare setjmp context for exception handling */
 605         if (sigsetjmp(cpu->jmp_env, 0) == 0) {
 606             /* if an exception is pending, we execute it here */
 607             if (cpu_handle_exception(cpu, &ret)) {
 608                 break;
 609             }
 610
 611             last_tb = NULL; /* forget the last executed TB after exception */
 612             cpu->tb_flushed = false; /* reset before first TB lookup */
 613             for(;;) {
 614                 cpu_handle_interrupt(cpu, &last_tb);
 615                 tb = tb_find_fast(cpu, &last_tb, tb_exit);
 616                 cpu_loop_exec_tb(cpu, tb, &last_tb, &tb_exit, &sc);
 617                 /* Try to align the host and virtual clocks
 618                    if the guest is in advance */
 619                 align_clocks(&sc, cpu);
 620             } /* for(;;) */
 621         } else {
 622 #if defined(__clang__) || !QEMU_GNUC_PREREQ(4, 6)
 623             /* Some compilers wrongly smash all local variables after
 624              * siglongjmp. There were bug reports for gcc 4.5.0 and clang.
 625              * Reload essential local variables here for those compilers.
 626              * Newer versions of gcc would complain about this code (-Wclobbered). */
 627             cpu = current_cpu;
 628             cc = CPU_GET_CLASS(cpu);
 629 #else /* buggy compiler */
 630             /* Assert that the compiler does not smash local variables. */
 631             g_assert(cpu == current_cpu);
 632             g_assert(cc == CPU_GET_CLASS(cpu));
 633 #endif /* buggy compiler */
 634             cpu->can_do_io = 1;
 635             tb_lock_reset();
 636         }
 637     } /* for(;;) */
 638
 639     cc->cpu_exec_exit(cpu);
 640     rcu_read_unlock();
 641
 642     /* fail safe : never use current_cpu outside cpu_exec() */
 643     current_cpu = NULL;
 644
 645     /* Does not need atomic_mb_set because a spurious wakeup is okay.  */
 646     atomic_set(&tcg_current_cpu, NULL);
 647     return ret;
 648 }