Re: [Qemu-devel] [PATCH v4 23/26] tcg: Emit prologue to the beginning of code_gen_buffer

[Aurelien Jarno <aurelien@aurel32.net>]

On 2015-09-30 15:09, Richard Henderson wrote:
> By putting the prologue at the end, we risk overwriting the
> prologue should our estimate of maximum TB size.  Given the
> two different placements of the call to tcg_prologue_init,
> move the high water mark computation into tcg_prologue_init.
> 
> Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
> Signed-off-by: Richard Henderson <rth@twiddle.net>
> ---
>  tcg/tcg.c       | 35 ++++++++++++++++++++++++++++-------
>  translate-all.c | 28 +++++++++-------------------
>  2 files changed, 37 insertions(+), 26 deletions(-)

Good idea to move it. I have done some experiments with putting slow
path "helpers" in the prologue, and I ended-up going over the 1024
bytes limits.

> diff --git a/tcg/tcg.c b/tcg/tcg.c
> index d3693b1..5609108 100644
> --- a/tcg/tcg.c
> +++ b/tcg/tcg.c
> @@ -363,17 +363,38 @@ void tcg_context_init(TCGContext *s)
>  
>  void tcg_prologue_init(TCGContext *s)
>  {
> -    /* init global prologue and epilogue */
> -    s->code_buf = s->code_gen_prologue;
> -    s->code_ptr = s->code_buf;
> +    size_t prologue_size, total_size;
> +    void *buf0, *buf1;
> +
> +    /* Put the prologue at the beginning of code_gen_buffer.  */
> +    buf0 = s->code_gen_buffer;
> +    s->code_ptr = buf0;
> +    s->code_buf = buf0;
> +    s->code_gen_prologue = buf0;
> +
> +    /* Generate the prologue.  */
>      tcg_target_qemu_prologue(s);
> -    flush_icache_range((uintptr_t)s->code_buf, (uintptr_t)s->code_ptr);
> +    buf1 = s->code_ptr;
> +    flush_icache_range((uintptr_t)buf0, (uintptr_t)buf1);
> +
> +    /* Deduct the prologue from the buffer.  */
> +    prologue_size = tcg_current_code_size(s);
> +    s->code_gen_ptr = buf1;
> +    s->code_gen_buffer = buf1;
> +    s->code_buf = buf1;
> +    total_size = s->code_gen_buffer_size - prologue_size;
> +    s->code_gen_buffer_size = total_size;
> +
> +    /* Compute a high-water mark, at which we voluntarily flush the
> +       buffer and start over.  */
> +    s->code_gen_buffer_max_size = total_size - TCG_MAX_OP_SIZE * OPC_BUF_SIZE;
> +
> +    tcg_register_jit(s->code_gen_buffer, total_size);

I am not sure why you moved this 2 lines there, I think they have more
their place in code_gen_alloc() so that the heuristics stay at the same
place. total_size is available in s->code_gen_buffer_size, so that
should be doable.

>  #ifdef DEBUG_DISAS
>      if (qemu_loglevel_mask(CPU_LOG_TB_OUT_ASM)) {
> -        size_t size = tcg_current_code_size(s);
> -        qemu_log("PROLOGUE: [size=%zu]\n", size);
> -        log_disas(s->code_buf, size);
> +        qemu_log("PROLOGUE: [size=%zu]\n", prologue_size);
> +        log_disas(buf0, prologue_size);
>          qemu_log("\n");
>          qemu_log_flush();
>      }
> diff --git a/translate-all.c b/translate-all.c
> index 3454f4e..0e8d176 100644
> --- a/translate-all.c
> +++ b/translate-all.c
> @@ -690,23 +690,15 @@ static inline void code_gen_alloc(size_t tb_size)
>      }
>  
>      qemu_madvise(tcg_ctx.code_gen_buffer, tcg_ctx.code_gen_buffer_size,
> -            QEMU_MADV_HUGEPAGE);
> -
> -    /* Steal room for the prologue at the end of the buffer.  This ensures
> -       (via the MAX_CODE_GEN_BUFFER_SIZE limits above) that direct branches
> -       from TB's to the prologue are going to be in range.  It also means
> -       that we don't need to mark (additional) portions of the data segment
> -       as executable.  */
> -    tcg_ctx.code_gen_prologue = tcg_ctx.code_gen_buffer +
> -            tcg_ctx.code_gen_buffer_size - 1024;
> -    tcg_ctx.code_gen_buffer_size -= 1024;
> -
> -    tcg_ctx.code_gen_buffer_max_size = tcg_ctx.code_gen_buffer_size -
> -        (TCG_MAX_OP_SIZE * OPC_BUF_SIZE);
> -    tcg_ctx.code_gen_max_blocks = tcg_ctx.code_gen_buffer_size /
> -            CODE_GEN_AVG_BLOCK_SIZE;
> -    tcg_ctx.tb_ctx.tbs =
> -            g_malloc(tcg_ctx.code_gen_max_blocks * sizeof(TranslationBlock));
> +                 QEMU_MADV_HUGEPAGE);
> +
> +    /* Estimate a good size for the number of TBs we can support.  We
> +       still haven't deducted the prologue from the buffer size here,
> +       but that's minimal and won't affect the estimate much.  */
> +    tcg_ctx.code_gen_max_blocks
> +        = tcg_ctx.code_gen_buffer_size / CODE_GEN_AVG_BLOCK_SIZE;
> +    tcg_ctx.tb_ctx.tbs = g_new(TranslationBlock, tcg_ctx.code_gen_max_blocks);
> +
>      qemu_mutex_init(&tcg_ctx.tb_ctx.tb_lock);
>  }
>  
> @@ -717,8 +709,6 @@ void tcg_exec_init(unsigned long tb_size)
>  {
>      cpu_gen_init();
>      code_gen_alloc(tb_size);
> -    tcg_ctx.code_gen_ptr = tcg_ctx.code_gen_buffer;
> -    tcg_register_jit(tcg_ctx.code_gen_buffer, tcg_ctx.code_gen_buffer_size);
>      page_init();
>  #if defined(CONFIG_SOFTMMU)
>      /* There's no guest base to take into account, so go ahead and

Otherwise the patch looks fine to me.

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net