Re: [PULL v2 10/10] tcg: Restart code generation when we run out of temps

[Roman Bolshakov <r.bolshakov@yadro.com>]

On Sun, Jan 24, 2021 at 08:11:22AM -1000, Richard Henderson wrote:
> Some large translation blocks can generate so many unique
> constants that we run out of temps to hold them.  In this
> case, longjmp back to the start of code generation and
> restart with a smaller translation block.
> 
> Buglink: https://bugs.launchpad.net/bugs/1912065
> Tested-by: BALATON Zoltan <balaton@eik.bme.hu>
> Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  include/tcg/tcg.h         |  3 +++
>  accel/tcg/translate-all.c | 15 ++++++++++++++-
>  tcg/tcg.c                 | 11 ++++++++---
>  3 files changed, 25 insertions(+), 4 deletions(-)
> 
> diff --git a/include/tcg/tcg.h b/include/tcg/tcg.h
> index c5a9d65d5f..0f0695e90d 100644
> --- a/include/tcg/tcg.h
> +++ b/include/tcg/tcg.h
> @@ -680,6 +680,9 @@ struct TCGContext {
>  
>      uint16_t gen_insn_end_off[TCG_MAX_INSNS];
>      target_ulong gen_insn_data[TCG_MAX_INSNS][TARGET_INSN_START_WORDS];
> +
> +    /* Exit to translator on overflow. */
> +    sigjmp_buf jmp_trans;
>  };
>  
>  static inline bool temp_readonly(TCGTemp *ts)
> diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
> index d09c187e0f..81d4c83f22 100644
> --- a/accel/tcg/translate-all.c
> +++ b/accel/tcg/translate-all.c
> @@ -1926,11 +1926,17 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
>      ti = profile_getclock();
>  #endif
>  
> +    gen_code_size = sigsetjmp(tcg_ctx->jmp_trans, 0);
> +    if (unlikely(gen_code_size != 0)) {
> +        goto error_return;
> +    }
> +
>      tcg_func_start(tcg_ctx);
>  
>      tcg_ctx->cpu = env_cpu(env);
>      gen_intermediate_code(cpu, tb, max_insns);
>      tcg_ctx->cpu = NULL;
> +    max_insns = tb->icount;
>  
>      trace_translate_block(tb, tb->pc, tb->tc.ptr);
>  
> @@ -1955,6 +1961,7 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
>  
>      gen_code_size = tcg_gen_code(tcg_ctx, tb);
>      if (unlikely(gen_code_size < 0)) {
> + error_return:
>          switch (gen_code_size) {
>          case -1:
>              /*
> @@ -1966,6 +1973,9 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
>               * flush the TBs, allocate a new TB, re-initialize it per
>               * above, and re-do the actual code generation.
>               */
> +            qemu_log_mask(CPU_LOG_TB_OP | CPU_LOG_TB_OP_OPT,
> +                          "Restarting code generation for "
> +                          "code_gen_buffer overflow\n");
>              goto buffer_overflow;
>  
>          case -2:
> @@ -1978,9 +1988,12 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
>               * Try again with half as many insns as we attempted this time.
>               * If a single insn overflows, there's a bug somewhere...
>               */
> -            max_insns = tb->icount;
>              assert(max_insns > 1);
>              max_insns /= 2;
> +            qemu_log_mask(CPU_LOG_TB_OP | CPU_LOG_TB_OP_OPT,
> +                          "Restarting code generation with "
> +                          "smaller translation block (max %d insns)\n",
> +                          max_insns);
>              goto tb_overflow;
>  
>          default:
> diff --git a/tcg/tcg.c b/tcg/tcg.c
> index 67b08f708d..9e1b0d73c7 100644
> --- a/tcg/tcg.c
> +++ b/tcg/tcg.c
> @@ -1205,18 +1205,23 @@ void tcg_func_start(TCGContext *s)
>      QSIMPLEQ_INIT(&s->labels);
>  }
>  
> -static inline TCGTemp *tcg_temp_alloc(TCGContext *s)
> +static TCGTemp *tcg_temp_alloc(TCGContext *s)
>  {
>      int n = s->nb_temps++;
> -    tcg_debug_assert(n < TCG_MAX_TEMPS);
> +
> +    if (n >= TCG_MAX_TEMPS) {
> +        /* Signal overflow, starting over with fewer guest insns. */
> +        siglongjmp(s->jmp_trans, -2);
> +    }
>      return memset(&s->temps[n], 0, sizeof(TCGTemp));
>  }
>  
> -static inline TCGTemp *tcg_global_alloc(TCGContext *s)
> +static TCGTemp *tcg_global_alloc(TCGContext *s)
>  {
>      TCGTemp *ts;
>  
>      tcg_debug_assert(s->nb_globals == s->nb_temps);
> +    tcg_debug_assert(s->nb_globals < TCG_MAX_TEMPS);
>      s->nb_globals++;
>      ts = tcg_temp_alloc(s);
>      ts->kind = TEMP_GLOBAL;
> -- 
> 2.25.1
> 
> 

Hi Richard,

Thanks for providing the fix.

Tested-by: Roman Bolshakov <r.bolshakov@yadro.com>

Regards,
Roman