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         /* check next page if needed */
 246         if (tb->page_addr[1] == -1) {
 247             return true;
 248         } else {
 249             tb_page_addr_t phys_page2;
 250             target_ulong virt_page2;
 251
 252             virt_page2 = (desc->pc & TARGET_PAGE_MASK) + TARGET_PAGE_SIZE;
 253             phys_page2 = get_page_addr_code(desc->env, virt_page2);
 254             if (tb->page_addr[1] == phys_page2) {
 255                 return true;
 256             }
 257         }
 258     }
 259     return false;
 260 }
 261
 262 static TranslationBlock *tb_find_physical(CPUState *cpu,
 263                                           target_ulong pc,
 264                                           target_ulong cs_base,
 265                                           uint32_t flags)
 266 {
 267     tb_page_addr_t phys_pc;
 268     struct tb_desc desc;
 269     uint32_t h;
 270
 271     desc.env = (CPUArchState *)cpu->env_ptr;
 272     desc.cs_base = cs_base;
 273     desc.flags = flags;
 274     desc.pc = pc;
 275     phys_pc = get_page_addr_code(desc.env, pc);
 276     desc.phys_page1 = phys_pc & TARGET_PAGE_MASK;
 277     h = tb_hash_func(phys_pc, pc, flags);
 278     return qht_lookup(&tcg_ctx.tb_ctx.htable, tb_cmp, &desc, h);
 279 }
 280
 281 static TranslationBlock *tb_find_slow(CPUState *cpu,
 282                                       target_ulong pc,
 283                                       target_ulong cs_base,
 284                                       uint32_t flags)
 285 {
 286     TranslationBlock *tb;
 287
 288     tb = tb_find_physical(cpu, pc, cs_base, flags);
 289     if (tb) {
 290         goto found;
 291     }
 292
 293 #ifdef CONFIG_USER_ONLY
 294     /* mmap_lock is needed by tb_gen_code, and mmap_lock must be
 295      * taken outside tb_lock.  Since we're momentarily dropping
 296      * tb_lock, there's a chance that our desired tb has been
 297      * translated.
 298      */
 299     tb_unlock();
 300     mmap_lock();
 301     tb_lock();
 302     tb = tb_find_physical(cpu, pc, cs_base, flags);
 303     if (tb) {
 304         mmap_unlock();
 305         goto found;
 306     }
 307 #endif
 308
 309     /* if no translated code available, then translate it now */
 310     tb = tb_gen_code(cpu, pc, cs_base, flags, 0);
 311
 312 #ifdef CONFIG_USER_ONLY
 313     mmap_unlock();
 314 #endif
 315
 316 found:
 317     /* we add the TB in the virtual pc hash table */
 318     cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)] = tb;
 319     return tb;
 320 }
 321
 322 static inline TranslationBlock *tb_find_fast(CPUState *cpu,
 323                                              TranslationBlock **last_tb,
 324                                              int tb_exit)
 325 {
 326     CPUArchState *env = (CPUArchState *)cpu->env_ptr;
 327     TranslationBlock *tb;
 328     target_ulong cs_base, pc;
 329     uint32_t flags;
 330
 331     /* we record a subset of the CPU state. It will
 332        always be the same before a given translated block
 333        is executed. */
 334     cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
 335     tb_lock();
 336     tb = cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)];
 337     if (unlikely(!tb || tb->pc != pc || tb->cs_base != cs_base ||
 338                  tb->flags != flags)) {
 339         tb = tb_find_slow(cpu, pc, cs_base, flags);
 340     }
 341     if (cpu->tb_flushed) {
 342         /* Ensure that no TB jump will be modified as the
 343          * translation buffer has been flushed.
 344          */
 345         *last_tb = NULL;
 346         cpu->tb_flushed = false;
 347     }
 348 #ifndef CONFIG_USER_ONLY
 349     /* We don't take care of direct jumps when address mapping changes in
 350      * system emulation. So it's not safe to make a direct jump to a TB
 351      * spanning two pages because the mapping for the second page can change.
 352      */
 353     if (tb->page_addr[1] != -1) {
 354         *last_tb = NULL;
 355     }
 356 #endif
 357     /* See if we can patch the calling TB. */
 358     if (*last_tb && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
 359         tb_add_jump(*last_tb, tb_exit, tb);
 360     }
 361     tb_unlock();
 362     return tb;
 363 }
 364
 365 static inline bool cpu_handle_halt(CPUState *cpu)
 366 {
 367     if (cpu->halted) {
 368 #if defined(TARGET_I386) && !defined(CONFIG_USER_ONLY)
 369         if ((cpu->interrupt_request & CPU_INTERRUPT_POLL)
 370             && replay_interrupt()) {
 371             X86CPU *x86_cpu = X86_CPU(cpu);
 372             apic_poll_irq(x86_cpu->apic_state);
 373             cpu_reset_interrupt(cpu, CPU_INTERRUPT_POLL);
 374         }
 375 #endif
 376         if (!cpu_has_work(cpu)) {
 377             current_cpu = NULL;
 378             return true;
 379         }
 380
 381         cpu->halted = 0;
 382     }
 383
 384     return false;
 385 }
 386
 387 static inline void cpu_handle_debug_exception(CPUState *cpu)
 388 {
 389     CPUClass *cc = CPU_GET_CLASS(cpu);
 390     CPUWatchpoint *wp;
 391
 392     if (!cpu->watchpoint_hit) {
 393         QTAILQ_FOREACH(wp, &cpu->watchpoints, entry) {
 394             wp->flags &= ~BP_WATCHPOINT_HIT;
 395         }
 396     }
 397
 398     cc->debug_excp_handler(cpu);
 399 }
 400
 401 static inline bool cpu_handle_exception(CPUState *cpu, int *ret)
 402 {
 403     if (cpu->exception_index >= 0) {
 404         if (cpu->exception_index >= EXCP_INTERRUPT) {
 405             /* exit request from the cpu execution loop */
 406             *ret = cpu->exception_index;
 407             if (*ret == EXCP_DEBUG) {
 408                 cpu_handle_debug_exception(cpu);
 409             }
 410             cpu->exception_index = -1;
 411             return true;
 412         } else {
 413 #if defined(CONFIG_USER_ONLY)
 414             /* if user mode only, we simulate a fake exception
 415                which will be handled outside the cpu execution
 416                loop */
 417 #if defined(TARGET_I386)
 418             CPUClass *cc = CPU_GET_CLASS(cpu);
 419             cc->do_interrupt(cpu);
 420 #endif
 421             *ret = cpu->exception_index;
 422             cpu->exception_index = -1;
 423             return true;
 424 #else
 425             if (replay_exception()) {
 426                 CPUClass *cc = CPU_GET_CLASS(cpu);
 427                 cc->do_interrupt(cpu);
 428                 cpu->exception_index = -1;
 429             } else if (!replay_has_interrupt()) {
 430                 /* give a chance to iothread in replay mode */
 431                 *ret = EXCP_INTERRUPT;
 432                 return true;
 433             }
 434 #endif
 435         }
 436 #ifndef CONFIG_USER_ONLY
 437     } else if (replay_has_exception()
 438                && cpu->icount_decr.u16.low + cpu->icount_extra == 0) {
 439         /* try to cause an exception pending in the log */
 440         TranslationBlock *last_tb = NULL; /* Avoid chaining TBs */
 441         cpu_exec_nocache(cpu, 1, tb_find_fast(cpu, &last_tb, 0), true);
 442         *ret = -1;
 443         return true;
 444 #endif
 445     }
 446
 447     return false;
 448 }
 449
 450 static inline void cpu_handle_interrupt(CPUState *cpu,
 451                                         TranslationBlock **last_tb)
 452 {
 453     CPUClass *cc = CPU_GET_CLASS(cpu);
 454     int interrupt_request = cpu->interrupt_request;
 455
 456     if (unlikely(interrupt_request)) {
 457         if (unlikely(cpu->singlestep_enabled & SSTEP_NOIRQ)) {
 458             /* Mask out external interrupts for this step. */
 459             interrupt_request &= ~CPU_INTERRUPT_SSTEP_MASK;
 460         }
 461         if (interrupt_request & CPU_INTERRUPT_DEBUG) {
 462             cpu->interrupt_request &= ~CPU_INTERRUPT_DEBUG;
 463             cpu->exception_index = EXCP_DEBUG;
 464             cpu_loop_exit(cpu);
 465         }
 466         if (replay_mode == REPLAY_MODE_PLAY && !replay_has_interrupt()) {
 467             /* Do nothing */
 468         } else if (interrupt_request & CPU_INTERRUPT_HALT) {
 469             replay_interrupt();
 470             cpu->interrupt_request &= ~CPU_INTERRUPT_HALT;
 471             cpu->halted = 1;
 472             cpu->exception_index = EXCP_HLT;
 473             cpu_loop_exit(cpu);
 474         }
 475 #if defined(TARGET_I386)
 476         else if (interrupt_request & CPU_INTERRUPT_INIT) {
 477             X86CPU *x86_cpu = X86_CPU(cpu);
 478             CPUArchState *env = &x86_cpu->env;
 479             replay_interrupt();
 480             cpu_svm_check_intercept_param(env, SVM_EXIT_INIT, 0);
 481             do_cpu_init(x86_cpu);
 482             cpu->exception_index = EXCP_HALTED;
 483             cpu_loop_exit(cpu);
 484         }
 485 #else
 486         else if (interrupt_request & CPU_INTERRUPT_RESET) {
 487             replay_interrupt();
 488             cpu_reset(cpu);
 489             cpu_loop_exit(cpu);
 490         }
 491 #endif
 492         /* The target hook has 3 exit conditions:
 493            False when the interrupt isn't processed,
 494            True when it is, and we should restart on a new TB,
 495            and via longjmp via cpu_loop_exit.  */
 496         else {
 497             replay_interrupt();
 498             if (cc->cpu_exec_interrupt(cpu, interrupt_request)) {
 499                 *last_tb = NULL;
 500             }
 501             /* The target hook may have updated the 'cpu->interrupt_request';
 502              * reload the 'interrupt_request' value */
 503             interrupt_request = cpu->interrupt_request;
 504         }
 505         if (interrupt_request & CPU_INTERRUPT_EXITTB) {
 506             cpu->interrupt_request &= ~CPU_INTERRUPT_EXITTB;
 507             /* ensure that no TB jump will be modified as
 508                the program flow was changed */
 509             *last_tb = NULL;
 510         }
 511     }
 512     if (unlikely(cpu->exit_request || replay_has_interrupt())) {
 513         cpu->exit_request = 0;
 514         cpu->exception_index = EXCP_INTERRUPT;
 515         cpu_loop_exit(cpu);
 516     }
 517 }
 518
 519 static inline void cpu_loop_exec_tb(CPUState *cpu, TranslationBlock *tb,
 520                                     TranslationBlock **last_tb, int *tb_exit,
 521                                     SyncClocks *sc)
 522 {
 523     uintptr_t ret;
 524
 525     if (unlikely(cpu->exit_request)) {
 526         return;
 527     }
 528
 529     trace_exec_tb(tb, tb->pc);
 530     ret = cpu_tb_exec(cpu, tb);
 531     *last_tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
 532     *tb_exit = ret & TB_EXIT_MASK;
 533     switch (*tb_exit) {
 534     case TB_EXIT_REQUESTED:
 535         /* Something asked us to stop executing
 536          * chained TBs; just continue round the main
 537          * loop. Whatever requested the exit will also
 538          * have set something else (eg exit_request or
 539          * interrupt_request) which we will handle
 540          * next time around the loop.  But we need to
 541          * ensure the tcg_exit_req read in generated code
 542          * comes before the next read of cpu->exit_request
 543          * or cpu->interrupt_request.
 544          */
 545         smp_rmb();
 546         *last_tb = NULL;
 547         break;
 548     case TB_EXIT_ICOUNT_EXPIRED:
 549     {
 550         /* Instruction counter expired.  */
 551 #ifdef CONFIG_USER_ONLY
 552         abort();
 553 #else
 554         int insns_left = cpu->icount_decr.u32;
 555         if (cpu->icount_extra && insns_left >= 0) {
 556             /* Refill decrementer and continue execution.  */
 557             cpu->icount_extra += insns_left;
 558             insns_left = MIN(0xffff, cpu->icount_extra);
 559             cpu->icount_extra -= insns_left;
 560             cpu->icount_decr.u16.low = insns_left;
 561         } else {
 562             if (insns_left > 0) {
 563                 /* Execute remaining instructions.  */
 564                 cpu_exec_nocache(cpu, insns_left, *last_tb, false);
 565                 align_clocks(sc, cpu);
 566             }
 567             cpu->exception_index = EXCP_INTERRUPT;
 568             *last_tb = NULL;
 569             cpu_loop_exit(cpu);
 570         }
 571         break;
 572 #endif
 573     }
 574     default:
 575         break;
 576     }
 577 }
 578
 579 /* main execution loop */
 580
 581 int cpu_exec(CPUState *cpu)
 582 {
 583     CPUClass *cc = CPU_GET_CLASS(cpu);
 584     int ret;
 585     SyncClocks sc;
 586
 587     /* replay_interrupt may need current_cpu */
 588     current_cpu = cpu;
 589
 590     if (cpu_handle_halt(cpu)) {
 591         return EXCP_HALTED;
 592     }
 593
 594     atomic_mb_set(&tcg_current_cpu, cpu);
 595     rcu_read_lock();
 596
 597     if (unlikely(atomic_mb_read(&exit_request))) {
 598         cpu->exit_request = 1;
 599     }
 600
 601     cc->cpu_exec_enter(cpu);
 602
 603     /* Calculate difference between guest clock and host clock.
 604      * This delay includes the delay of the last cycle, so
 605      * what we have to do is sleep until it is 0. As for the
 606      * advance/delay we gain here, we try to fix it next time.
 607      */
 608     init_delay_params(&sc, cpu);
 609
 610     for(;;) {
 611         TranslationBlock *tb, *last_tb;
 612         int tb_exit = 0;
 613
 614         /* prepare setjmp context for exception handling */
 615         if (sigsetjmp(cpu->jmp_env, 0) == 0) {
 616             /* if an exception is pending, we execute it here */
 617             if (cpu_handle_exception(cpu, &ret)) {
 618                 break;
 619             }
 620
 621             last_tb = NULL; /* forget the last executed TB after exception */
 622             cpu->tb_flushed = false; /* reset before first TB lookup */
 623             for(;;) {
 624                 cpu_handle_interrupt(cpu, &last_tb);
 625                 tb = tb_find_fast(cpu, &last_tb, tb_exit);
 626                 cpu_loop_exec_tb(cpu, tb, &last_tb, &tb_exit, &sc);
 627                 /* Try to align the host and virtual clocks
 628                    if the guest is in advance */
 629                 align_clocks(&sc, cpu);
 630             } /* for(;;) */
 631         } else {
 632 #if defined(__clang__) || !QEMU_GNUC_PREREQ(4, 6)
 633             /* Some compilers wrongly smash all local variables after
 634              * siglongjmp. There were bug reports for gcc 4.5.0 and clang.
 635              * Reload essential local variables here for those compilers.
 636              * Newer versions of gcc would complain about this code (-Wclobbered). */
 637             cpu = current_cpu;
 638             cc = CPU_GET_CLASS(cpu);
 639 #else /* buggy compiler */
 640             /* Assert that the compiler does not smash local variables. */
 641             g_assert(cpu == current_cpu);
 642             g_assert(cc == CPU_GET_CLASS(cpu));
 643 #endif /* buggy compiler */
 644             cpu->can_do_io = 1;
 645             tb_lock_reset();
 646         }
 647     } /* for(;;) */
 648
 649     cc->cpu_exec_exit(cpu);
 650     rcu_read_unlock();
 651
 652     /* fail safe : never use current_cpu outside cpu_exec() */
 653     current_cpu = NULL;
 654
 655     /* Does not need atomic_mb_set because a spurious wakeup is okay.  */
 656     atomic_set(&tcg_current_cpu, NULL);
 657     return ret;
 658 }