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 "config.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
  28 /* -icount align implementation. */
  29
  30 typedef struct SyncClocks {
  31     int64_t diff_clk;
  32     int64_t last_cpu_icount;
  33     int64_t realtime_clock;
  34 } SyncClocks;
  35
  36 #if !defined(CONFIG_USER_ONLY)
  37 /* Allow the guest to have a max 3ms advance.
  38  * The difference between the 2 clocks could therefore
  39  * oscillate around 0.
  40  */
  41 #define VM_CLOCK_ADVANCE 3000000
  42 #define THRESHOLD_REDUCE 1.5
  43 #define MAX_DELAY_PRINT_RATE 2000000000LL
  44 #define MAX_NB_PRINTS 100
  45
  46 static void align_clocks(SyncClocks *sc, const CPUState *cpu)
  47 {
  48     int64_t cpu_icount;
  49
  50     if (!icount_align_option) {
  51         return;
  52     }
  53
  54     cpu_icount = cpu->icount_extra + cpu->icount_decr.u16.low;
  55     sc->diff_clk += cpu_icount_to_ns(sc->last_cpu_icount - cpu_icount);
  56     sc->last_cpu_icount = cpu_icount;
  57
  58     if (sc->diff_clk > VM_CLOCK_ADVANCE) {
  59 #ifndef _WIN32
  60         struct timespec sleep_delay, rem_delay;
  61         sleep_delay.tv_sec = sc->diff_clk / 1000000000LL;
  62         sleep_delay.tv_nsec = sc->diff_clk % 1000000000LL;
  63         if (nanosleep(&sleep_delay, &rem_delay) < 0) {
  64             sc->diff_clk -= (sleep_delay.tv_sec - rem_delay.tv_sec) * 1000000000LL;
  65             sc->diff_clk -= sleep_delay.tv_nsec - rem_delay.tv_nsec;
  66         } else {
  67             sc->diff_clk = 0;
  68         }
  69 #else
  70         Sleep(sc->diff_clk / SCALE_MS);
  71         sc->diff_clk = 0;
  72 #endif
  73     }
  74 }
  75
  76 static void print_delay(const SyncClocks *sc)
  77 {
  78     static float threshold_delay;
  79     static int64_t last_realtime_clock;
  80     static int nb_prints;
  81
  82     if (icount_align_option &&
  83         sc->realtime_clock - last_realtime_clock >= MAX_DELAY_PRINT_RATE &&
  84         nb_prints < MAX_NB_PRINTS) {
  85         if ((-sc->diff_clk / (float)1000000000LL > threshold_delay) ||
  86             (-sc->diff_clk / (float)1000000000LL <
  87              (threshold_delay - THRESHOLD_REDUCE))) {
  88             threshold_delay = (-sc->diff_clk / 1000000000LL) + 1;
  89             printf("Warning: The guest is now late by %.1f to %.1f seconds\n",
  90                    threshold_delay - 1,
  91                    threshold_delay);
  92             nb_prints++;
  93             last_realtime_clock = sc->realtime_clock;
  94         }
  95     }
  96 }
  97
  98 static void init_delay_params(SyncClocks *sc,
  99                               const CPUState *cpu)
 100 {
 101     if (!icount_align_option) {
 102         return;
 103     }
 104     sc->realtime_clock = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
 105     sc->diff_clk = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) -
 106                    sc->realtime_clock +
 107                    cpu_get_clock_offset();
 108     sc->last_cpu_icount = cpu->icount_extra + cpu->icount_decr.u16.low;
 109     if (sc->diff_clk < max_delay) {
 110         max_delay = sc->diff_clk;
 111     }
 112     if (sc->diff_clk > max_advance) {
 113         max_advance = sc->diff_clk;
 114     }
 115
 116     /* Print every 2s max if the guest is late. We limit the number
 117        of printed messages to NB_PRINT_MAX(currently 100) */
 118     print_delay(sc);
 119 }
 120 #else
 121 static void align_clocks(SyncClocks *sc, const CPUState *cpu)
 122 {
 123 }
 124
 125 static void init_delay_params(SyncClocks *sc, const CPUState *cpu)
 126 {
 127 }
 128 #endif /* CONFIG USER ONLY */
 129
 130 void cpu_loop_exit(CPUState *cpu)
 131 {
 132     cpu->current_tb = NULL;
 133     siglongjmp(cpu->jmp_env, 1);
 134 }
 135
 136 /* exit the current TB from a signal handler. The host registers are
 137    restored in a state compatible with the CPU emulator
 138  */
 139 #if defined(CONFIG_SOFTMMU)
 140 void cpu_resume_from_signal(CPUState *cpu, void *puc)
 141 {
 142     /* XXX: restore cpu registers saved in host registers */
 143
 144     cpu->exception_index = -1;
 145     siglongjmp(cpu->jmp_env, 1);
 146 }
 147 #endif
 148
 149 /* Execute a TB, and fix up the CPU state afterwards if necessary */
 150 static inline tcg_target_ulong cpu_tb_exec(CPUState *cpu, uint8_t *tb_ptr)
 151 {
 152     CPUArchState *env = cpu->env_ptr;
 153     uintptr_t next_tb;
 154
 155 #if defined(DEBUG_DISAS)
 156     if (qemu_loglevel_mask(CPU_LOG_TB_CPU)) {
 157 #if defined(TARGET_I386)
 158         log_cpu_state(cpu, CPU_DUMP_CCOP);
 159 #elif defined(TARGET_M68K)
 160         /* ??? Should not modify env state for dumping.  */
 161         cpu_m68k_flush_flags(env, env->cc_op);
 162         env->cc_op = CC_OP_FLAGS;
 163         env->sr = (env->sr & 0xffe0) | env->cc_dest | (env->cc_x << 4);
 164         log_cpu_state(cpu, 0);
 165 #else
 166         log_cpu_state(cpu, 0);
 167 #endif
 168     }
 169 #endif /* DEBUG_DISAS */
 170
 171     next_tb = tcg_qemu_tb_exec(env, tb_ptr);
 172     trace_exec_tb_exit((void *) (next_tb & ~TB_EXIT_MASK),
 173                        next_tb & TB_EXIT_MASK);
 174
 175     if ((next_tb & TB_EXIT_MASK) > TB_EXIT_IDX1) {
 176         /* We didn't start executing this TB (eg because the instruction
 177          * counter hit zero); we must restore the guest PC to the address
 178          * of the start of the TB.
 179          */
 180         CPUClass *cc = CPU_GET_CLASS(cpu);
 181         TranslationBlock *tb = (TranslationBlock *)(next_tb & ~TB_EXIT_MASK);
 182         if (cc->synchronize_from_tb) {
 183             cc->synchronize_from_tb(cpu, tb);
 184         } else {
 185             assert(cc->set_pc);
 186             cc->set_pc(cpu, tb->pc);
 187         }
 188     }
 189     if ((next_tb & TB_EXIT_MASK) == 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 next_tb;
 196 }
 197
 198 /* Execute the code without caching the generated code. An interpreter
 199    could be used if available. */
 200 static void cpu_exec_nocache(CPUArchState *env, int max_cycles,
 201                              TranslationBlock *orig_tb)
 202 {
 203     CPUState *cpu = ENV_GET_CPU(env);
 204     TranslationBlock *tb;
 205
 206     /* Should never happen.
 207        We only end up here when an existing TB is too long.  */
 208     if (max_cycles > CF_COUNT_MASK)
 209         max_cycles = CF_COUNT_MASK;
 210
 211     tb = tb_gen_code(cpu, orig_tb->pc, orig_tb->cs_base, orig_tb->flags,
 212                      max_cycles);
 213     cpu->current_tb = tb;
 214     /* execute the generated code */
 215     trace_exec_tb_nocache(tb, tb->pc);
 216     cpu_tb_exec(cpu, tb->tc_ptr);
 217     cpu->current_tb = NULL;
 218     tb_phys_invalidate(tb, -1);
 219     tb_free(tb);
 220 }
 221
 222 static TranslationBlock *tb_find_slow(CPUArchState *env,
 223                                       target_ulong pc,
 224                                       target_ulong cs_base,
 225                                       uint64_t flags)
 226 {
 227     CPUState *cpu = ENV_GET_CPU(env);
 228     TranslationBlock *tb, **ptb1;
 229     unsigned int h;
 230     tb_page_addr_t phys_pc, phys_page1;
 231     target_ulong virt_page2;
 232
 233     tcg_ctx.tb_ctx.tb_invalidated_flag = 0;
 234
 235     /* find translated block using physical mappings */
 236     phys_pc = get_page_addr_code(env, pc);
 237     phys_page1 = phys_pc & TARGET_PAGE_MASK;
 238     h = tb_phys_hash_func(phys_pc);
 239     ptb1 = &tcg_ctx.tb_ctx.tb_phys_hash[h];
 240     for(;;) {
 241         tb = *ptb1;
 242         if (!tb)
 243             goto not_found;
 244         if (tb->pc == pc &&
 245             tb->page_addr[0] == phys_page1 &&
 246             tb->cs_base == cs_base &&
 247             tb->flags == flags) {
 248             /* check next page if needed */
 249             if (tb->page_addr[1] != -1) {
 250                 tb_page_addr_t phys_page2;
 251
 252                 virt_page2 = (pc & TARGET_PAGE_MASK) +
 253                     TARGET_PAGE_SIZE;
 254                 phys_page2 = get_page_addr_code(env, virt_page2);
 255                 if (tb->page_addr[1] == phys_page2)
 256                     goto found;
 257             } else {
 258                 goto found;
 259             }
 260         }
 261         ptb1 = &tb->phys_hash_next;
 262     }
 263  not_found:
 264    /* if no translated code available, then translate it now */
 265     tb = tb_gen_code(cpu, pc, cs_base, flags, 0);
 266
 267  found:
 268     /* Move the last found TB to the head of the list */
 269     if (likely(*ptb1)) {
 270         *ptb1 = tb->phys_hash_next;
 271         tb->phys_hash_next = tcg_ctx.tb_ctx.tb_phys_hash[h];
 272         tcg_ctx.tb_ctx.tb_phys_hash[h] = tb;
 273     }
 274     /* we add the TB in the virtual pc hash table */
 275     cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)] = tb;
 276     return tb;
 277 }
 278
 279 static inline TranslationBlock *tb_find_fast(CPUArchState *env)
 280 {
 281     CPUState *cpu = ENV_GET_CPU(env);
 282     TranslationBlock *tb;
 283     target_ulong cs_base, pc;
 284     int flags;
 285
 286     /* we record a subset of the CPU state. It will
 287        always be the same before a given translated block
 288        is executed. */
 289     cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
 290     tb = cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)];
 291     if (unlikely(!tb || tb->pc != pc || tb->cs_base != cs_base ||
 292                  tb->flags != flags)) {
 293         tb = tb_find_slow(env, pc, cs_base, flags);
 294     }
 295     return tb;
 296 }
 297
 298 static void cpu_handle_debug_exception(CPUArchState *env)
 299 {
 300     CPUState *cpu = ENV_GET_CPU(env);
 301     CPUClass *cc = CPU_GET_CLASS(cpu);
 302     CPUWatchpoint *wp;
 303
 304     if (!cpu->watchpoint_hit) {
 305         QTAILQ_FOREACH(wp, &cpu->watchpoints, entry) {
 306             wp->flags &= ~BP_WATCHPOINT_HIT;
 307         }
 308     }
 309
 310     cc->debug_excp_handler(cpu);
 311 }
 312
 313 /* main execution loop */
 314
 315 volatile sig_atomic_t exit_request;
 316
 317 int cpu_exec(CPUArchState *env)
 318 {
 319     CPUState *cpu = ENV_GET_CPU(env);
 320     CPUClass *cc = CPU_GET_CLASS(cpu);
 321 #ifdef TARGET_I386
 322     X86CPU *x86_cpu = X86_CPU(cpu);
 323 #endif
 324     int ret, interrupt_request;
 325     TranslationBlock *tb;
 326     uint8_t *tc_ptr;
 327     uintptr_t next_tb;
 328     SyncClocks sc;
 329
 330     /* This must be volatile so it is not trashed by longjmp() */
 331     volatile bool have_tb_lock = false;
 332
 333     if (cpu->halted) {
 334         if (!cpu_has_work(cpu)) {
 335             return EXCP_HALTED;
 336         }
 337
 338         cpu->halted = 0;
 339     }
 340
 341     current_cpu = cpu;
 342
 343     /* As long as current_cpu is null, up to the assignment just above,
 344      * requests by other threads to exit the execution loop are expected to
 345      * be issued using the exit_request global. We must make sure that our
 346      * evaluation of the global value is performed past the current_cpu
 347      * value transition point, which requires a memory barrier as well as
 348      * an instruction scheduling constraint on modern architectures.  */
 349     smp_mb();
 350
 351     if (unlikely(exit_request)) {
 352         cpu->exit_request = 1;
 353     }
 354
 355     cc->cpu_exec_enter(cpu);
 356     cpu->exception_index = -1;
 357
 358     /* Calculate difference between guest clock and host clock.
 359      * This delay includes the delay of the last cycle, so
 360      * what we have to do is sleep until it is 0. As for the
 361      * advance/delay we gain here, we try to fix it next time.
 362      */
 363     init_delay_params(&sc, cpu);
 364
 365     /* prepare setjmp context for exception handling */
 366     for(;;) {
 367         if (sigsetjmp(cpu->jmp_env, 0) == 0) {
 368             /* if an exception is pending, we execute it here */
 369             if (cpu->exception_index >= 0) {
 370                 if (cpu->exception_index >= EXCP_INTERRUPT) {
 371                     /* exit request from the cpu execution loop */
 372                     ret = cpu->exception_index;
 373                     if (ret == EXCP_DEBUG) {
 374                         cpu_handle_debug_exception(env);
 375                     }
 376                     break;
 377                 } else {
 378 #if defined(CONFIG_USER_ONLY)
 379                     /* if user mode only, we simulate a fake exception
 380                        which will be handled outside the cpu execution
 381                        loop */
 382 #if defined(TARGET_I386)
 383                     cc->do_interrupt(cpu);
 384 #endif
 385                     ret = cpu->exception_index;
 386                     break;
 387 #else
 388                     cc->do_interrupt(cpu);
 389                     cpu->exception_index = -1;
 390 #endif
 391                 }
 392             }
 393
 394             next_tb = 0; /* force lookup of first TB */
 395             for(;;) {
 396                 interrupt_request = cpu->interrupt_request;
 397                 if (unlikely(interrupt_request)) {
 398                     if (unlikely(cpu->singlestep_enabled & SSTEP_NOIRQ)) {
 399                         /* Mask out external interrupts for this step. */
 400                         interrupt_request &= ~CPU_INTERRUPT_SSTEP_MASK;
 401                     }
 402                     if (interrupt_request & CPU_INTERRUPT_DEBUG) {
 403                         cpu->interrupt_request &= ~CPU_INTERRUPT_DEBUG;
 404                         cpu->exception_index = EXCP_DEBUG;
 405                         cpu_loop_exit(cpu);
 406                     }
 407 #if defined(TARGET_ARM) || defined(TARGET_SPARC) || defined(TARGET_MIPS) || \
 408     defined(TARGET_PPC) || defined(TARGET_ALPHA) || defined(TARGET_CRIS) || \
 409     defined(TARGET_MICROBLAZE) || defined(TARGET_LM32) ||                   \
 410     defined(TARGET_UNICORE32) || defined(TARGET_TRICORE)
 411                     if (interrupt_request & CPU_INTERRUPT_HALT) {
 412                         cpu->interrupt_request &= ~CPU_INTERRUPT_HALT;
 413                         cpu->halted = 1;
 414                         cpu->exception_index = EXCP_HLT;
 415                         cpu_loop_exit(cpu);
 416                     }
 417 #endif
 418 #if defined(TARGET_I386)
 419                     if (interrupt_request & CPU_INTERRUPT_INIT) {
 420                         cpu_svm_check_intercept_param(env, SVM_EXIT_INIT, 0);
 421                         do_cpu_init(x86_cpu);
 422                         cpu->exception_index = EXCP_HALTED;
 423                         cpu_loop_exit(cpu);
 424                     }
 425 #else
 426                     if (interrupt_request & CPU_INTERRUPT_RESET) {
 427                         cpu_reset(cpu);
 428                     }
 429 #endif
 430 #if defined(TARGET_I386)
 431 #if !defined(CONFIG_USER_ONLY)
 432                     if (interrupt_request & CPU_INTERRUPT_POLL) {
 433                         cpu->interrupt_request &= ~CPU_INTERRUPT_POLL;
 434                         apic_poll_irq(x86_cpu->apic_state);
 435                     }
 436 #endif
 437                     if (interrupt_request & CPU_INTERRUPT_SIPI) {
 438                             do_cpu_sipi(x86_cpu);
 439                     } else if (env->hflags2 & HF2_GIF_MASK) {
 440                         if ((interrupt_request & CPU_INTERRUPT_SMI) &&
 441                             !(env->hflags & HF_SMM_MASK)) {
 442                             cpu_svm_check_intercept_param(env, SVM_EXIT_SMI,
 443                                                           0);
 444                             cpu->interrupt_request &= ~CPU_INTERRUPT_SMI;
 445                             do_smm_enter(x86_cpu);
 446                             next_tb = 0;
 447                         } else if ((interrupt_request & CPU_INTERRUPT_NMI) &&
 448                                    !(env->hflags2 & HF2_NMI_MASK)) {
 449                             cpu->interrupt_request &= ~CPU_INTERRUPT_NMI;
 450                             env->hflags2 |= HF2_NMI_MASK;
 451                             do_interrupt_x86_hardirq(env, EXCP02_NMI, 1);
 452                             next_tb = 0;
 453                         } else if (interrupt_request & CPU_INTERRUPT_MCE) {
 454                             cpu->interrupt_request &= ~CPU_INTERRUPT_MCE;
 455                             do_interrupt_x86_hardirq(env, EXCP12_MCHK, 0);
 456                             next_tb = 0;
 457                         } else if ((interrupt_request & CPU_INTERRUPT_HARD) &&
 458                                    (((env->hflags2 & HF2_VINTR_MASK) &&
 459                                      (env->hflags2 & HF2_HIF_MASK)) ||
 460                                     (!(env->hflags2 & HF2_VINTR_MASK) &&
 461                                      (env->eflags & IF_MASK &&
 462                                       !(env->hflags & HF_INHIBIT_IRQ_MASK))))) {
 463                             int intno;
 464                             cpu_svm_check_intercept_param(env, SVM_EXIT_INTR,
 465                                                           0);
 466                             cpu->interrupt_request &= ~(CPU_INTERRUPT_HARD |
 467                                                         CPU_INTERRUPT_VIRQ);
 468                             intno = cpu_get_pic_interrupt(env);
 469                             qemu_log_mask(CPU_LOG_TB_IN_ASM, "Servicing hardware INT=0x%02x\n", intno);
 470                             do_interrupt_x86_hardirq(env, intno, 1);
 471                             /* ensure that no TB jump will be modified as
 472                                the program flow was changed */
 473                             next_tb = 0;
 474 #if !defined(CONFIG_USER_ONLY)
 475                         } else if ((interrupt_request & CPU_INTERRUPT_VIRQ) &&
 476                                    (env->eflags & IF_MASK) &&
 477                                    !(env->hflags & HF_INHIBIT_IRQ_MASK)) {
 478                             int intno;
 479                             /* FIXME: this should respect TPR */
 480                             cpu_svm_check_intercept_param(env, SVM_EXIT_VINTR,
 481                                                           0);
 482                             intno = ldl_phys(cpu->as,
 483                                              env->vm_vmcb
 484                                              + offsetof(struct vmcb,
 485                                                         control.int_vector));
 486                             qemu_log_mask(CPU_LOG_TB_IN_ASM, "Servicing virtual hardware INT=0x%02x\n", intno);
 487                             do_interrupt_x86_hardirq(env, intno, 1);
 488                             cpu->interrupt_request &= ~CPU_INTERRUPT_VIRQ;
 489                             next_tb = 0;
 490 #endif
 491                         }
 492                     }
 493 #elif defined(TARGET_PPC)
 494                     if (interrupt_request & CPU_INTERRUPT_HARD) {
 495                         ppc_hw_interrupt(env);
 496                         if (env->pending_interrupts == 0) {
 497                             cpu->interrupt_request &= ~CPU_INTERRUPT_HARD;
 498                         }
 499                         next_tb = 0;
 500                     }
 501 #elif defined(TARGET_LM32)
 502                     if ((interrupt_request & CPU_INTERRUPT_HARD)
 503                         && (env->ie & IE_IE)) {
 504                         cpu->exception_index = EXCP_IRQ;
 505                         cc->do_interrupt(cpu);
 506                         next_tb = 0;
 507                     }
 508 #endif
 509                     /* The target hook has 3 exit conditions:
 510                        False when the interrupt isn't processed,
 511                        True when it is, and we should restart on a new TB,
 512                        and via longjmp via cpu_loop_exit.  */
 513                     if (cc->cpu_exec_interrupt(cpu, interrupt_request)) {
 514                         next_tb = 0;
 515                     }
 516                     /* Don't use the cached interrupt_request value,
 517                        do_interrupt may have updated the EXITTB flag. */
 518                     if (cpu->interrupt_request & CPU_INTERRUPT_EXITTB) {
 519                         cpu->interrupt_request &= ~CPU_INTERRUPT_EXITTB;
 520                         /* ensure that no TB jump will be modified as
 521                            the program flow was changed */
 522                         next_tb = 0;
 523                     }
 524                 }
 525                 if (unlikely(cpu->exit_request)) {
 526                     cpu->exit_request = 0;
 527                     cpu->exception_index = EXCP_INTERRUPT;
 528                     cpu_loop_exit(cpu);
 529                 }
 530                 spin_lock(&tcg_ctx.tb_ctx.tb_lock);
 531                 have_tb_lock = true;
 532                 tb = tb_find_fast(env);
 533                 /* Note: we do it here to avoid a gcc bug on Mac OS X when
 534                    doing it in tb_find_slow */
 535                 if (tcg_ctx.tb_ctx.tb_invalidated_flag) {
 536                     /* as some TB could have been invalidated because
 537                        of memory exceptions while generating the code, we
 538                        must recompute the hash index here */
 539                     next_tb = 0;
 540                     tcg_ctx.tb_ctx.tb_invalidated_flag = 0;
 541                 }
 542                 if (qemu_loglevel_mask(CPU_LOG_EXEC)) {
 543                     qemu_log("Trace %p [" TARGET_FMT_lx "] %s\n",
 544                              tb->tc_ptr, tb->pc, lookup_symbol(tb->pc));
 545                 }
 546                 /* see if we can patch the calling TB. When the TB
 547                    spans two pages, we cannot safely do a direct
 548                    jump. */
 549                 if (next_tb != 0 && tb->page_addr[1] == -1) {
 550                     tb_add_jump((TranslationBlock *)(next_tb & ~TB_EXIT_MASK),
 551                                 next_tb & TB_EXIT_MASK, tb);
 552                 }
 553                 have_tb_lock = false;
 554                 spin_unlock(&tcg_ctx.tb_ctx.tb_lock);
 555
 556                 /* cpu_interrupt might be called while translating the
 557                    TB, but before it is linked into a potentially
 558                    infinite loop and becomes env->current_tb. Avoid
 559                    starting execution if there is a pending interrupt. */
 560                 cpu->current_tb = tb;
 561                 barrier();
 562                 if (likely(!cpu->exit_request)) {
 563                     trace_exec_tb(tb, tb->pc);
 564                     tc_ptr = tb->tc_ptr;
 565                     /* execute the generated code */
 566                     next_tb = cpu_tb_exec(cpu, tc_ptr);
 567                     switch (next_tb & TB_EXIT_MASK) {
 568                     case TB_EXIT_REQUESTED:
 569                         /* Something asked us to stop executing
 570                          * chained TBs; just continue round the main
 571                          * loop. Whatever requested the exit will also
 572                          * have set something else (eg exit_request or
 573                          * interrupt_request) which we will handle
 574                          * next time around the loop.
 575                          */
 576                         tb = (TranslationBlock *)(next_tb & ~TB_EXIT_MASK);
 577                         next_tb = 0;
 578                         break;
 579                     case TB_EXIT_ICOUNT_EXPIRED:
 580                     {
 581                         /* Instruction counter expired.  */
 582                         int insns_left;
 583                         tb = (TranslationBlock *)(next_tb & ~TB_EXIT_MASK);
 584                         insns_left = cpu->icount_decr.u32;
 585                         if (cpu->icount_extra && insns_left >= 0) {
 586                             /* Refill decrementer and continue execution.  */
 587                             cpu->icount_extra += insns_left;
 588                             if (cpu->icount_extra > 0xffff) {
 589                                 insns_left = 0xffff;
 590                             } else {
 591                                 insns_left = cpu->icount_extra;
 592                             }
 593                             cpu->icount_extra -= insns_left;
 594                             cpu->icount_decr.u16.low = insns_left;
 595                         } else {
 596                             if (insns_left > 0) {
 597                                 /* Execute remaining instructions.  */
 598                                 cpu_exec_nocache(env, insns_left, tb);
 599                                 align_clocks(&sc, cpu);
 600                             }
 601                             cpu->exception_index = EXCP_INTERRUPT;
 602                             next_tb = 0;
 603                             cpu_loop_exit(cpu);
 604                         }
 605                         break;
 606                     }
 607                     default:
 608                         break;
 609                     }
 610                 }
 611                 cpu->current_tb = NULL;
 612                 /* Try to align the host and virtual clocks
 613                    if the guest is in advance */
 614                 align_clocks(&sc, cpu);
 615                 /* reset soft MMU for next block (it can currently
 616                    only be set by a memory fault) */
 617             } /* for(;;) */
 618         } else {
 619             /* Reload env after longjmp - the compiler may have smashed all
 620              * local variables as longjmp is marked 'noreturn'. */
 621             cpu = current_cpu;
 622             env = cpu->env_ptr;
 623             cc = CPU_GET_CLASS(cpu);
 624 #ifdef TARGET_I386
 625             x86_cpu = X86_CPU(cpu);
 626 #endif
 627             if (have_tb_lock) {
 628                 spin_unlock(&tcg_ctx.tb_ctx.tb_lock);
 629                 have_tb_lock = false;
 630             }
 631         }
 632     } /* for(;;) */
 633
 634     cc->cpu_exec_exit(cpu);
 635
 636     /* fail safe : never use current_cpu outside cpu_exec() */
 637     current_cpu = NULL;
 638     return ret;
 639 }