Re: [Qemu-devel] [PATCH v4 4/5] tcg: Clean up from 'next_tb'

["Alex Bennée" <alex.bennee@linaro.org>]

Sergey Fedorov <sergey.fedorov@linaro.org> writes:

> From: Sergey Fedorov <serge.fdrv@gmail.com>
>
> The value returned from tcg_qemu_tb_exec() is the value passed to the
> corresponding tcg_gen_exit_tb() at translation time of the last TB
> attempted to execute. It is a little confusing to store it in a variable
> named 'next_tb'. In fact, it is a combination of 4-byte aligned pointer
> and additional information in its two least significant bits. Break it
> down right away into two variables named 'last_tb' and 'tb_exit' which
> are a pointer to the last TB attempted to execute and the TB exit
> reason, correspondingly. This simplifies the code and improves its
> readability.
>
> Correct a misleading documentation comment for tcg_qemu_tb_exec() and
> fix logging in cpu_tb_exec(). Also rename a misleading 'next_tb' in
> another couple of places.
>
> Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
> Signed-off-by: Sergey Fedorov <sergey.fedorov@linaro.org>
> ---
>  cpu-exec.c   | 59 ++++++++++++++++++++++++++++++++---------------------------
>  tcg/tcg.h    | 19 ++++++++++---------
>  tci.c        |  6 +++---
>  trace-events |  2 +-
>  4 files changed, 46 insertions(+), 40 deletions(-)
>
> diff --git a/cpu-exec.c b/cpu-exec.c
> index 36942340d7e3..08b5c21c29bb 100644
> --- a/cpu-exec.c
> +++ b/cpu-exec.c
> @@ -136,7 +136,9 @@ static void init_delay_params(SyncClocks *sc, const CPUState *cpu)
>  static inline tcg_target_ulong cpu_tb_exec(CPUState *cpu, TranslationBlock *itb)
>  {
>      CPUArchState *env = cpu->env_ptr;
> -    uintptr_t next_tb;
> +    uintptr_t ret;
> +    TranslationBlock *last_tb;
> +    int tb_exit;
>      uint8_t *tb_ptr = itb->tc_ptr;
>
>      qemu_log_mask_and_addr(CPU_LOG_EXEC, itb->pc,
> @@ -160,36 +162,37 @@ static inline tcg_target_ulong cpu_tb_exec(CPUState *cpu, TranslationBlock *itb)
>  #endif /* DEBUG_DISAS */
>
>      cpu->can_do_io = !use_icount;
> -    next_tb = tcg_qemu_tb_exec(env, tb_ptr);
> +    ret = tcg_qemu_tb_exec(env, tb_ptr);
>      cpu->can_do_io = 1;
> -    trace_exec_tb_exit((void *) (next_tb & ~TB_EXIT_MASK),
> -                       next_tb & TB_EXIT_MASK);
> +    last_tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
> +    tb_exit = ret & TB_EXIT_MASK;
> +    trace_exec_tb_exit(last_tb, tb_exit);
>
> -    if ((next_tb & TB_EXIT_MASK) > TB_EXIT_IDX1) {
> +    if (tb_exit > TB_EXIT_IDX1) {
>          /* We didn't start executing this TB (eg because the instruction
>           * counter hit zero); we must restore the guest PC to the address
>           * of the start of the TB.
>           */
>          CPUClass *cc = CPU_GET_CLASS(cpu);
> -        TranslationBlock *tb = (TranslationBlock *)(next_tb & ~TB_EXIT_MASK);
> -        qemu_log_mask_and_addr(CPU_LOG_EXEC, itb->pc,
> +        qemu_log_mask_and_addr(CPU_LOG_EXEC, last_tb->pc,
>                                 "Stopped execution of TB chain before %p ["
>                                 TARGET_FMT_lx "] %s\n",
> -                               itb->tc_ptr, itb->pc, lookup_symbol(itb->pc));
> +                               last_tb->tc_ptr, last_tb->pc,
> +                               lookup_symbol(last_tb->pc));
>          if (cc->synchronize_from_tb) {
> -            cc->synchronize_from_tb(cpu, tb);
> +            cc->synchronize_from_tb(cpu, last_tb);
>          } else {
>              assert(cc->set_pc);
> -            cc->set_pc(cpu, tb->pc);
> +            cc->set_pc(cpu, last_tb->pc);
>          }
>      }
> -    if ((next_tb & TB_EXIT_MASK) == TB_EXIT_REQUESTED) {
> +    if (tb_exit == TB_EXIT_REQUESTED) {
>          /* We were asked to stop executing TBs (probably a pending
>           * interrupt. We've now stopped, so clear the flag.
>           */
>          cpu->tcg_exit_req = 0;
>      }
> -    return next_tb;
> +    return ret;
>  }
>
>  #ifndef CONFIG_USER_ONLY
> @@ -358,8 +361,8 @@ int cpu_exec(CPUState *cpu)
>      CPUArchState *env = &x86_cpu->env;
>  #endif
>      int ret, interrupt_request;
> -    TranslationBlock *tb;
> -    uintptr_t next_tb;
> +    TranslationBlock *tb, *last_tb;
> +    int tb_exit = 0;
>      SyncClocks sc;
>
>      /* replay_interrupt may need current_cpu */
> @@ -442,7 +445,7 @@ int cpu_exec(CPUState *cpu)
>  #endif
>              }
>
> -            next_tb = 0; /* force lookup of first TB */
> +            last_tb = NULL; /* forget the last executed TB after exception */
>              for(;;) {
>                  interrupt_request = cpu->interrupt_request;
>                  if (unlikely(interrupt_request)) {
> @@ -487,7 +490,7 @@ int cpu_exec(CPUState *cpu)
>                      else {
>                          replay_interrupt();
>                          if (cc->cpu_exec_interrupt(cpu, interrupt_request)) {
> -                            next_tb = 0;
> +                            last_tb = NULL;
>                          }
>                      }
>                      /* Don't use the cached interrupt_request value,
> @@ -496,7 +499,7 @@ int cpu_exec(CPUState *cpu)
>                          cpu->interrupt_request &= ~CPU_INTERRUPT_EXITTB;
>                          /* ensure that no TB jump will be modified as
>                             the program flow was changed */
> -                        next_tb = 0;
> +                        last_tb = NULL;
>                      }
>                  }
>                  if (unlikely(cpu->exit_request
> @@ -513,25 +516,27 @@ int cpu_exec(CPUState *cpu)
>                      /* as some TB could have been invalidated because
>                         of memory exceptions while generating the code, we
>                         must recompute the hash index here */
> -                    next_tb = 0;
> +                    last_tb = NULL;
>                      tcg_ctx.tb_ctx.tb_invalidated_flag = 0;
>                  }
>                  /* see if we can patch the calling TB. When the TB
>                     spans two pages, we cannot safely do a direct
>                     jump. */
> -                if (next_tb != 0 && tb->page_addr[1] == -1
> +                if (last_tb != NULL && tb->page_addr[1] == -1

We can shortcut non-null tests:

                if (last_tb && tb->page_addr[1] == -1

>                      && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
> -                    tb_add_jump((TranslationBlock *)(next_tb & ~TB_EXIT_MASK),
> -                                next_tb & TB_EXIT_MASK, tb);
> +                    tb_add_jump(last_tb, tb_exit, tb);
>                  }
>                  tb_unlock();
>                  if (likely(!cpu->exit_request)) {
> +                    uintptr_t ret;
>                      trace_exec_tb(tb, tb->pc);
>                      /* execute the generated code */
>                      cpu->current_tb = tb;
> -                    next_tb = cpu_tb_exec(cpu, tb);
> +                    ret = cpu_tb_exec(cpu, tb);
>                      cpu->current_tb = NULL;
> -                    switch (next_tb & TB_EXIT_MASK) {
> +                    last_tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
> +                    tb_exit = ret & TB_EXIT_MASK;
> +                    switch (tb_exit) {
>                      case TB_EXIT_REQUESTED:
>                          /* Something asked us to stop executing
>                           * chained TBs; just continue round the main
> @@ -544,7 +549,7 @@ int cpu_exec(CPUState *cpu)
>                           * or cpu->interrupt_request.
>                           */
>                          smp_rmb();
> -                        next_tb = 0;
> +                        last_tb = NULL;
>                          break;
>                      case TB_EXIT_ICOUNT_EXPIRED:
>                      {
> @@ -562,12 +567,12 @@ int cpu_exec(CPUState *cpu)
>                          } else {
>                              if (insns_left > 0) {
>                                  /* Execute remaining instructions.  */
> -                                tb = (TranslationBlock *)(next_tb & ~TB_EXIT_MASK);
> -                                cpu_exec_nocache(cpu, insns_left, tb, false);
> +                                cpu_exec_nocache(cpu, insns_left,
> +                                                 last_tb, false);
>                                  align_clocks(&sc, cpu);
>                              }
>                              cpu->exception_index = EXCP_INTERRUPT;
> -                            next_tb = 0;
> +                            last_tb = NULL;
>                              cpu_loop_exit(cpu);
>                          }
>                          break;
> diff --git a/tcg/tcg.h b/tcg/tcg.h
> index 40c8fbe2ae64..cd58ea42c61d 100644
> --- a/tcg/tcg.h
> +++ b/tcg/tcg.h
> @@ -919,7 +919,7 @@ static inline unsigned get_mmuidx(TCGMemOpIdx oi)
>
>  /**
>   * tcg_qemu_tb_exec:
> - * @env: CPUArchState * for the CPU
> + * @env: pointer to CPUArchState for the CPU
>   * @tb_ptr: address of generated code for the TB to execute
>   *
>   * Start executing code from a given translation block.
> @@ -930,30 +930,31 @@ static inline unsigned get_mmuidx(TCGMemOpIdx oi)
>   * which has not yet been directly linked, or an asynchronous
>   * event such as an interrupt needs handling.
>   *
> - * The return value is a pointer to the next TB to execute
> - * (if known; otherwise zero). This pointer is assumed to be
> - * 4-aligned, and the bottom two bits are used to return further
> - * information:
> + * Return: The return value is the value passed to the corresponding
> + * tcg_gen_exit_tb() at translation time of the last TB attempted to execute.
> + * The value is either zero or a 4-byte aligned pointer to that TB combined
> + * with additional information in its two least significant bits. The
> + * additional information is encoded as follows:
>   *  0, 1: the link between this TB and the next is via the specified
>   *        TB index (0 or 1). That is, we left the TB via (the equivalent
>   *        of) "goto_tb <index>". The main loop uses this to determine
>   *        how to link the TB just executed to the next.
>   *  2:    we are using instruction counting code generation, and we
>   *        did not start executing this TB because the instruction counter
> - *        would hit zero midway through it. In this case the next-TB pointer
> + *        would hit zero midway through it. In this case the pointer
>   *        returned is the TB we were about to execute, and the caller must
>   *        arrange to execute the remaining count of instructions.
>   *  3:    we stopped because the CPU's exit_request flag was set
>   *        (usually meaning that there is an interrupt that needs to be
> - *        handled). The next-TB pointer returned is the TB we were
> - *        about to execute when we noticed the pending exit request.
> + *        handled). The pointer returned is the TB we were about to execute
> + *        when we noticed the pending exit request.
>   *
>   * If the bottom two bits indicate an exit-via-index then the CPU
>   * state is correctly synchronised and ready for execution of the next
>   * TB (and in particular the guest PC is the address to execute next).
>   * Otherwise, we gave up on execution of this TB before it started, and
>   * the caller must fix up the CPU state by calling the CPU's
> - * synchronize_from_tb() method with the next-TB pointer we return (falling
> + * synchronize_from_tb() method with the TB pointer we return (falling
>   * back to calling the CPU's set_pc method with tb->pb if no
>   * synchronize_from_tb() method exists).
>   *
> diff --git a/tci.c b/tci.c
> index 82705fe77295..8af97d618d3f 100644
> --- a/tci.c
> +++ b/tci.c
> @@ -467,7 +467,7 @@ uintptr_t tcg_qemu_tb_exec(CPUArchState *env, uint8_t *tb_ptr)
>  {
>      long tcg_temps[CPU_TEMP_BUF_NLONGS];
>      uintptr_t sp_value = (uintptr_t)(tcg_temps + CPU_TEMP_BUF_NLONGS);
> -    uintptr_t next_tb = 0;
> +    uintptr_t ret = 0;
>
>      tci_reg[TCG_AREG0] = (tcg_target_ulong)env;
>      tci_reg[TCG_REG_CALL_STACK] = sp_value;
> @@ -1085,7 +1085,7 @@ uintptr_t tcg_qemu_tb_exec(CPUArchState *env, uint8_t *tb_ptr)
>              /* QEMU specific operations. */
>
>          case INDEX_op_exit_tb:
> -            next_tb = *(uint64_t *)tb_ptr;
> +            ret = *(uint64_t *)tb_ptr;
>              goto exit;
>              break;
>          case INDEX_op_goto_tb:
> @@ -1240,5 +1240,5 @@ uintptr_t tcg_qemu_tb_exec(CPUArchState *env, uint8_t *tb_ptr)
>          tci_assert(tb_ptr == old_code_ptr + op_size);
>      }
>  exit:
> -    return next_tb;
> +    return ret;
>  }
> diff --git a/trace-events b/trace-events
> index 83507438789b..ef4da73e19f6 100644
> --- a/trace-events
> +++ b/trace-events
> @@ -1615,7 +1615,7 @@ kvm_failed_spr_get(int str, const char *msg) "Warning: Unable to retrieve SPR %d
>  # cpu-exec.c
>  disable exec_tb(void *tb, uintptr_t pc) "tb:%p pc=0x%"PRIxPTR
>  disable exec_tb_nocache(void *tb, uintptr_t pc) "tb:%p pc=0x%"PRIxPTR
> -disable exec_tb_exit(void *next_tb, unsigned int flags) "tb:%p flags=%x"
> +disable exec_tb_exit(void *last_tb, unsigned int flags) "tb:%p flags=%x"
>
>  # translate-all.c
>  translate_block(void *tb, uintptr_t pc, uint8_t *tb_code) "tb:%p, pc:0x%"PRIxPTR", tb_code:%p"


--
Alex Bennée