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 [%d: " TARGET_FMT_lx "] %s\n",
 147                            itb->tc_ptr, cpu->cpu_index, itb->pc,
 148                            lookup_symbol(itb->pc));
 149
 150 #if defined(DEBUG_DISAS)
 151     if (qemu_loglevel_mask(CPU_LOG_TB_CPU)
 152         && qemu_log_in_addr_range(itb->pc)) {
 153         qemu_log_lock();
 154 #if defined(TARGET_I386)
 155         log_cpu_state(cpu, CPU_DUMP_CCOP);
 156 #else
 157         log_cpu_state(cpu, 0);
 158 #endif
 159         qemu_log_unlock();
 160     }
 161 #endif /* DEBUG_DISAS */
 162
 163     cpu->can_do_io = !use_icount;
 164     ret = tcg_qemu_tb_exec(env, tb_ptr);
 165     cpu->can_do_io = 1;
 166     last_tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
 167     tb_exit = ret & TB_EXIT_MASK;
 168     trace_exec_tb_exit(last_tb, tb_exit);
 169
 170     if (tb_exit > TB_EXIT_IDX1) {
 171         /* We didn't start executing this TB (eg because the instruction
 172          * counter hit zero); we must restore the guest PC to the address
 173          * of the start of the TB.
 174          */
 175         CPUClass *cc = CPU_GET_CLASS(cpu);
 176         qemu_log_mask_and_addr(CPU_LOG_EXEC, last_tb->pc,
 177                                "Stopped execution of TB chain before %p ["
 178                                TARGET_FMT_lx "] %s\n",
 179                                last_tb->tc_ptr, last_tb->pc,
 180                                lookup_symbol(last_tb->pc));
 181         if (cc->synchronize_from_tb) {
 182             cc->synchronize_from_tb(cpu, last_tb);
 183         } else {
 184             assert(cc->set_pc);
 185             cc->set_pc(cpu, last_tb->pc);
 186         }
 187     }
 188     if (tb_exit == TB_EXIT_REQUESTED) {
 189         /* We were asked to stop executing TBs (probably a pending
 190          * interrupt. We've now stopped, so clear the flag.
 191          */
 192         atomic_set(&cpu->tcg_exit_req, 0);
 193     }
 194     return ret;
 195 }
 196
 197 #ifndef CONFIG_USER_ONLY
 198 /* Execute the code without caching the generated code. An interpreter
 199    could be used if available. */
 200 static void cpu_exec_nocache(CPUState *cpu, int max_cycles,
 201                              TranslationBlock *orig_tb, bool ignore_icount)
 202 {
 203     TranslationBlock *tb;
 204
 205     /* Should never happen.
 206        We only end up here when an existing TB is too long.  */
 207     if (max_cycles > CF_COUNT_MASK)
 208         max_cycles = CF_COUNT_MASK;
 209
 210     tb_lock();
 211     tb = tb_gen_code(cpu, orig_tb->pc, orig_tb->cs_base, orig_tb->flags,
 212                      max_cycles | CF_NOCACHE
 213                          | (ignore_icount ? CF_IGNORE_ICOUNT : 0));
 214     tb->orig_tb = orig_tb;
 215     tb_unlock();
 216
 217     /* execute the generated code */
 218     trace_exec_tb_nocache(tb, tb->pc);
 219     cpu_tb_exec(cpu, tb);
 220
 221     tb_lock();
 222     tb_phys_invalidate(tb, -1);
 223     tb_free(tb);
 224     tb_unlock();
 225 }
 226 #endif
 227
 228 static void cpu_exec_step(CPUState *cpu)
 229 {
 230     CPUArchState *env = (CPUArchState *)cpu->env_ptr;
 231     TranslationBlock *tb;
 232     target_ulong cs_base, pc;
 233     uint32_t flags;
 234
 235     cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
 236     tb = tb_gen_code(cpu, pc, cs_base, flags,
 237                      1 | CF_NOCACHE | CF_IGNORE_ICOUNT);
 238     tb->orig_tb = NULL;
 239     /* execute the generated code */
 240     trace_exec_tb_nocache(tb, pc);
 241     cpu_tb_exec(cpu, tb);
 242     tb_phys_invalidate(tb, -1);
 243     tb_free(tb);
 244 }
 245
 246 void cpu_exec_step_atomic(CPUState *cpu)
 247 {
 248     start_exclusive();
 249
 250     /* Since we got here, we know that parallel_cpus must be true.  */
 251     parallel_cpus = false;
 252     cpu_exec_step(cpu);
 253     parallel_cpus = true;
 254
 255     end_exclusive();
 256 }
 257
 258 struct tb_desc {
 259     target_ulong pc;
 260     target_ulong cs_base;
 261     CPUArchState *env;
 262     tb_page_addr_t phys_page1;
 263     uint32_t flags;
 264 };
 265
 266 static bool tb_cmp(const void *p, const void *d)
 267 {
 268     const TranslationBlock *tb = p;
 269     const struct tb_desc *desc = d;
 270
 271     if (tb->pc == desc->pc &&
 272         tb->page_addr[0] == desc->phys_page1 &&
 273         tb->cs_base == desc->cs_base &&
 274         tb->flags == desc->flags &&
 275         !atomic_read(&tb->invalid)) {
 276         /* check next page if needed */
 277         if (tb->page_addr[1] == -1) {
 278             return true;
 279         } else {
 280             tb_page_addr_t phys_page2;
 281             target_ulong virt_page2;
 282
 283             virt_page2 = (desc->pc & TARGET_PAGE_MASK) + TARGET_PAGE_SIZE;
 284             phys_page2 = get_page_addr_code(desc->env, virt_page2);
 285             if (tb->page_addr[1] == phys_page2) {
 286                 return true;
 287             }
 288         }
 289     }
 290     return false;
 291 }
 292
 293 static TranslationBlock *tb_htable_lookup(CPUState *cpu,
 294                                           target_ulong pc,
 295                                           target_ulong cs_base,
 296                                           uint32_t flags)
 297 {
 298     tb_page_addr_t phys_pc;
 299     struct tb_desc desc;
 300     uint32_t h;
 301
 302     desc.env = (CPUArchState *)cpu->env_ptr;
 303     desc.cs_base = cs_base;
 304     desc.flags = flags;
 305     desc.pc = pc;
 306     phys_pc = get_page_addr_code(desc.env, pc);
 307     desc.phys_page1 = phys_pc & TARGET_PAGE_MASK;
 308     h = tb_hash_func(phys_pc, pc, flags);
 309     return qht_lookup(&tcg_ctx.tb_ctx.htable, tb_cmp, &desc, h);
 310 }
 311
 312 static inline TranslationBlock *tb_find(CPUState *cpu,
 313                                         TranslationBlock *last_tb,
 314                                         int tb_exit)
 315 {
 316     CPUArchState *env = (CPUArchState *)cpu->env_ptr;
 317     TranslationBlock *tb;
 318     target_ulong cs_base, pc;
 319     uint32_t flags;
 320     bool have_tb_lock = false;
 321
 322     /* we record a subset of the CPU state. It will
 323        always be the same before a given translated block
 324        is executed. */
 325     cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
 326     tb = atomic_rcu_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)]);
 327     if (unlikely(!tb || tb->pc != pc || tb->cs_base != cs_base ||
 328                  tb->flags != flags)) {
 329         tb = tb_htable_lookup(cpu, pc, cs_base, flags);
 330         if (!tb) {
 331
 332             /* mmap_lock is needed by tb_gen_code, and mmap_lock must be
 333              * taken outside tb_lock. As system emulation is currently
 334              * single threaded the locks are NOPs.
 335              */
 336             mmap_lock();
 337             tb_lock();
 338             have_tb_lock = true;
 339
 340             /* There's a chance that our desired tb has been translated while
 341              * taking the locks so we check again inside the lock.
 342              */
 343             tb = tb_htable_lookup(cpu, pc, cs_base, flags);
 344             if (!tb) {
 345                 /* if no translated code available, then translate it now */
 346                 tb = tb_gen_code(cpu, pc, cs_base, flags, 0);
 347             }
 348
 349             mmap_unlock();
 350         }
 351
 352         /* We add the TB in the virtual pc hash table for the fast lookup */
 353         atomic_set(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)], tb);
 354     }
 355 #ifndef CONFIG_USER_ONLY
 356     /* We don't take care of direct jumps when address mapping changes in
 357      * system emulation. So it's not safe to make a direct jump to a TB
 358      * spanning two pages because the mapping for the second page can change.
 359      */
 360     if (tb->page_addr[1] != -1) {
 361         last_tb = NULL;
 362     }
 363 #endif
 364     /* See if we can patch the calling TB. */
 365     if (last_tb && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
 366         if (!have_tb_lock) {
 367             tb_lock();
 368             have_tb_lock = true;
 369         }
 370         if (!tb->invalid) {
 371             tb_add_jump(last_tb, tb_exit, tb);
 372         }
 373     }
 374     if (have_tb_lock) {
 375         tb_unlock();
 376     }
 377     return tb;
 378 }
 379
 380 static inline bool cpu_handle_halt(CPUState *cpu)
 381 {
 382     if (cpu->halted) {
 383 #if defined(TARGET_I386) && !defined(CONFIG_USER_ONLY)
 384         if ((cpu->interrupt_request & CPU_INTERRUPT_POLL)
 385             && replay_interrupt()) {
 386             X86CPU *x86_cpu = X86_CPU(cpu);
 387             apic_poll_irq(x86_cpu->apic_state);
 388             cpu_reset_interrupt(cpu, CPU_INTERRUPT_POLL);
 389         }
 390 #endif
 391         if (!cpu_has_work(cpu)) {
 392             current_cpu = NULL;
 393             return true;
 394         }
 395
 396         cpu->halted = 0;
 397     }
 398
 399     return false;
 400 }
 401
 402 static inline void cpu_handle_debug_exception(CPUState *cpu)
 403 {
 404     CPUClass *cc = CPU_GET_CLASS(cpu);
 405     CPUWatchpoint *wp;
 406
 407     if (!cpu->watchpoint_hit) {
 408         QTAILQ_FOREACH(wp, &cpu->watchpoints, entry) {
 409             wp->flags &= ~BP_WATCHPOINT_HIT;
 410         }
 411     }
 412
 413     cc->debug_excp_handler(cpu);
 414 }
 415
 416 static inline bool cpu_handle_exception(CPUState *cpu, int *ret)
 417 {
 418     if (cpu->exception_index >= 0) {
 419         if (cpu->exception_index >= EXCP_INTERRUPT) {
 420             /* exit request from the cpu execution loop */
 421             *ret = cpu->exception_index;
 422             if (*ret == EXCP_DEBUG) {
 423                 cpu_handle_debug_exception(cpu);
 424             }
 425             cpu->exception_index = -1;
 426             return true;
 427         } else {
 428 #if defined(CONFIG_USER_ONLY)
 429             /* if user mode only, we simulate a fake exception
 430                which will be handled outside the cpu execution
 431                loop */
 432 #if defined(TARGET_I386)
 433             CPUClass *cc = CPU_GET_CLASS(cpu);
 434             cc->do_interrupt(cpu);
 435 #endif
 436             *ret = cpu->exception_index;
 437             cpu->exception_index = -1;
 438             return true;
 439 #else
 440             if (replay_exception()) {
 441                 CPUClass *cc = CPU_GET_CLASS(cpu);
 442                 cc->do_interrupt(cpu);
 443                 cpu->exception_index = -1;
 444             } else if (!replay_has_interrupt()) {
 445                 /* give a chance to iothread in replay mode */
 446                 *ret = EXCP_INTERRUPT;
 447                 return true;
 448             }
 449 #endif
 450         }
 451 #ifndef CONFIG_USER_ONLY
 452     } else if (replay_has_exception()
 453                && cpu->icount_decr.u16.low + cpu->icount_extra == 0) {
 454         /* try to cause an exception pending in the log */
 455         cpu_exec_nocache(cpu, 1, tb_find(cpu, NULL, 0), true);
 456         *ret = -1;
 457         return true;
 458 #endif
 459     }
 460
 461     return false;
 462 }
 463
 464 static inline void cpu_handle_interrupt(CPUState *cpu,
 465                                         TranslationBlock **last_tb)
 466 {
 467     CPUClass *cc = CPU_GET_CLASS(cpu);
 468     int interrupt_request = cpu->interrupt_request;
 469
 470     if (unlikely(interrupt_request)) {
 471         if (unlikely(cpu->singlestep_enabled & SSTEP_NOIRQ)) {
 472             /* Mask out external interrupts for this step. */
 473             interrupt_request &= ~CPU_INTERRUPT_SSTEP_MASK;
 474         }
 475         if (interrupt_request & CPU_INTERRUPT_DEBUG) {
 476             cpu->interrupt_request &= ~CPU_INTERRUPT_DEBUG;
 477             cpu->exception_index = EXCP_DEBUG;
 478             cpu_loop_exit(cpu);
 479         }
 480         if (replay_mode == REPLAY_MODE_PLAY && !replay_has_interrupt()) {
 481             /* Do nothing */
 482         } else if (interrupt_request & CPU_INTERRUPT_HALT) {
 483             replay_interrupt();
 484             cpu->interrupt_request &= ~CPU_INTERRUPT_HALT;
 485             cpu->halted = 1;
 486             cpu->exception_index = EXCP_HLT;
 487             cpu_loop_exit(cpu);
 488         }
 489 #if defined(TARGET_I386)
 490         else if (interrupt_request & CPU_INTERRUPT_INIT) {
 491             X86CPU *x86_cpu = X86_CPU(cpu);
 492             CPUArchState *env = &x86_cpu->env;
 493             replay_interrupt();
 494             cpu_svm_check_intercept_param(env, SVM_EXIT_INIT, 0);
 495             do_cpu_init(x86_cpu);
 496             cpu->exception_index = EXCP_HALTED;
 497             cpu_loop_exit(cpu);
 498         }
 499 #else
 500         else if (interrupt_request & CPU_INTERRUPT_RESET) {
 501             replay_interrupt();
 502             cpu_reset(cpu);
 503             cpu_loop_exit(cpu);
 504         }
 505 #endif
 506         /* The target hook has 3 exit conditions:
 507            False when the interrupt isn't processed,
 508            True when it is, and we should restart on a new TB,
 509            and via longjmp via cpu_loop_exit.  */
 510         else {
 511             if (cc->cpu_exec_interrupt(cpu, interrupt_request)) {
 512                 replay_interrupt();
 513                 *last_tb = NULL;
 514             }
 515             /* The target hook may have updated the 'cpu->interrupt_request';
 516              * reload the 'interrupt_request' value */
 517             interrupt_request = cpu->interrupt_request;
 518         }
 519         if (interrupt_request & CPU_INTERRUPT_EXITTB) {
 520             cpu->interrupt_request &= ~CPU_INTERRUPT_EXITTB;
 521             /* ensure that no TB jump will be modified as
 522                the program flow was changed */
 523             *last_tb = NULL;
 524         }
 525     }
 526     if (unlikely(atomic_read(&cpu->exit_request) || replay_has_interrupt())) {
 527         atomic_set(&cpu->exit_request, 0);
 528         cpu->exception_index = EXCP_INTERRUPT;
 529         cpu_loop_exit(cpu);
 530     }
 531 }
 532
 533 static inline void cpu_loop_exec_tb(CPUState *cpu, TranslationBlock *tb,
 534                                     TranslationBlock **last_tb, int *tb_exit,
 535                                     SyncClocks *sc)
 536 {
 537     uintptr_t ret;
 538
 539     if (unlikely(atomic_read(&cpu->exit_request))) {
 540         return;
 541     }
 542
 543     trace_exec_tb(tb, tb->pc);
 544     ret = cpu_tb_exec(cpu, tb);
 545     *last_tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
 546     *tb_exit = ret & TB_EXIT_MASK;
 547     switch (*tb_exit) {
 548     case TB_EXIT_REQUESTED:
 549         /* Something asked us to stop executing
 550          * chained TBs; just continue round the main
 551          * loop. Whatever requested the exit will also
 552          * have set something else (eg exit_request or
 553          * interrupt_request) which we will handle
 554          * next time around the loop.  But we need to
 555          * ensure the tcg_exit_req read in generated code
 556          * comes before the next read of cpu->exit_request
 557          * or cpu->interrupt_request.
 558          */
 559         smp_rmb();
 560         *last_tb = NULL;
 561         break;
 562     case TB_EXIT_ICOUNT_EXPIRED:
 563     {
 564         /* Instruction counter expired.  */
 565 #ifdef CONFIG_USER_ONLY
 566         abort();
 567 #else
 568         int insns_left = cpu->icount_decr.u32;
 569         if (cpu->icount_extra && insns_left >= 0) {
 570             /* Refill decrementer and continue execution.  */
 571             cpu->icount_extra += insns_left;
 572             insns_left = MIN(0xffff, cpu->icount_extra);
 573             cpu->icount_extra -= insns_left;
 574             cpu->icount_decr.u16.low = insns_left;
 575         } else {
 576             if (insns_left > 0) {
 577                 /* Execute remaining instructions.  */
 578                 cpu_exec_nocache(cpu, insns_left, *last_tb, false);
 579                 align_clocks(sc, cpu);
 580             }
 581             cpu->exception_index = EXCP_INTERRUPT;
 582             *last_tb = NULL;
 583             cpu_loop_exit(cpu);
 584         }
 585         break;
 586 #endif
 587     }
 588     default:
 589         break;
 590     }
 591 }
 592
 593 /* main execution loop */
 594
 595 int cpu_exec(CPUState *cpu)
 596 {
 597     CPUClass *cc = CPU_GET_CLASS(cpu);
 598     int ret;
 599     SyncClocks sc;
 600
 601     /* replay_interrupt may need current_cpu */
 602     current_cpu = cpu;
 603
 604     if (cpu_handle_halt(cpu)) {
 605         return EXCP_HALTED;
 606     }
 607
 608     atomic_mb_set(&tcg_current_cpu, cpu);
 609     rcu_read_lock();
 610
 611     if (unlikely(atomic_mb_read(&exit_request))) {
 612         cpu->exit_request = 1;
 613     }
 614
 615     cc->cpu_exec_enter(cpu);
 616
 617     /* Calculate difference between guest clock and host clock.
 618      * This delay includes the delay of the last cycle, so
 619      * what we have to do is sleep until it is 0. As for the
 620      * advance/delay we gain here, we try to fix it next time.
 621      */
 622     init_delay_params(&sc, cpu);
 623
 624     for(;;) {
 625         /* prepare setjmp context for exception handling */
 626         if (sigsetjmp(cpu->jmp_env, 0) == 0) {
 627             TranslationBlock *tb, *last_tb = NULL;
 628             int tb_exit = 0;
 629
 630             /* if an exception is pending, we execute it here */
 631             if (cpu_handle_exception(cpu, &ret)) {
 632                 break;
 633             }
 634
 635             for(;;) {
 636                 cpu_handle_interrupt(cpu, &last_tb);
 637                 tb = tb_find(cpu, last_tb, tb_exit);
 638                 cpu_loop_exec_tb(cpu, tb, &last_tb, &tb_exit, &sc);
 639                 /* Try to align the host and virtual clocks
 640                    if the guest is in advance */
 641                 align_clocks(&sc, cpu);
 642             } /* for(;;) */
 643         } else {
 644 #if defined(__clang__) || !QEMU_GNUC_PREREQ(4, 6)
 645             /* Some compilers wrongly smash all local variables after
 646              * siglongjmp. There were bug reports for gcc 4.5.0 and clang.
 647              * Reload essential local variables here for those compilers.
 648              * Newer versions of gcc would complain about this code (-Wclobbered). */
 649             cpu = current_cpu;
 650             cc = CPU_GET_CLASS(cpu);
 651 #else /* buggy compiler */
 652             /* Assert that the compiler does not smash local variables. */
 653             g_assert(cpu == current_cpu);
 654             g_assert(cc == CPU_GET_CLASS(cpu));
 655 #endif /* buggy compiler */
 656             cpu->can_do_io = 1;
 657             tb_lock_reset();
 658         }
 659     } /* for(;;) */
 660
 661     cc->cpu_exec_exit(cpu);
 662     rcu_read_unlock();
 663
 664     /* fail safe : never use current_cpu outside cpu_exec() */
 665     current_cpu = NULL;
 666
 667     /* Does not need atomic_mb_set because a spurious wakeup is okay.  */
 668     atomic_set(&tcg_current_cpu, NULL);
 669     return ret;
 670 }