Re: [Qemu-devel] [PATCH v3 04/11] tcg: Prepare safe access to tb_flushed out of tb_lock

[Alex Bennée]

Sergey Fedorov <address@hidden> writes:

> From: Sergey Fedorov <address@hidden>
>
> Ensure atomicity of CPU's 'tb_flushed' access for future translation
> block lookup out of 'tb_lock'.
>
> This field can only be touched from another thread by tb_flush() in user
> mode emulation. So the only access to be atomic is:
>  * a single write in tb_flush();
>  * reads/writes out of 'tb_lock'.

It might worth mentioning the barrier here.

>
> In future, before enabling MTTCG in system mode, tb_flush() must be safe
> and this field becomes unnecessary.
>
> Signed-off-by: Sergey Fedorov <address@hidden>
> Signed-off-by: Sergey Fedorov <address@hidden>
> ---
>  cpu-exec.c      | 16 +++++++---------
>  translate-all.c |  4 ++--
>  2 files changed, 9 insertions(+), 11 deletions(-)
>
> diff --git a/cpu-exec.c b/cpu-exec.c
> index d6178eab71d4..c973e3b85922 100644
> --- a/cpu-exec.c
> +++ b/cpu-exec.c
> @@ -338,13 +338,6 @@ static inline TranslationBlock *tb_find_fast(CPUState 
> *cpu,
>                   tb->flags != flags)) {
>          tb = tb_find_slow(cpu, pc, cs_base, flags);
>      }
> -    if (cpu->tb_flushed) {
> -        /* Ensure that no TB jump will be modified as the
> -         * translation buffer has been flushed.
> -         */
> -        *last_tb = NULL;
> -        cpu->tb_flushed = false;
> -    }
>  #ifndef CONFIG_USER_ONLY
>      /* We don't take care of direct jumps when address mapping changes in
>       * system emulation. So it's not safe to make a direct jump to a TB
> @@ -356,7 +349,12 @@ static inline TranslationBlock *tb_find_fast(CPUState 
> *cpu,
>  #endif
>      /* See if we can patch the calling TB. */
>      if (last_tb && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
> -        tb_add_jump(last_tb, tb_exit, tb);
> +        /* Check if translation buffer has been flushed */
> +        if (cpu->tb_flushed) {
> +            cpu->tb_flushed = false;
> +        } else {
> +            tb_add_jump(last_tb, tb_exit, tb);
> +        }
>      }
>      tb_unlock();
>      return tb;
> @@ -618,7 +616,7 @@ int cpu_exec(CPUState *cpu)
>              }
>
>              last_tb = NULL; /* forget the last executed TB after exception */
> -            cpu->tb_flushed = false; /* reset before first TB lookup */
> +            atomic_mb_set(&cpu->tb_flushed, false); /* reset before first TB 
> lookup */
>              for(;;) {
>                  cpu_handle_interrupt(cpu, &last_tb);
>                  tb = tb_find_fast(cpu, last_tb, tb_exit);
> diff --git a/translate-all.c b/translate-all.c
> index fdf520a86d68..788fed1e0765 100644
> --- a/translate-all.c
> +++ b/translate-all.c
> @@ -845,7 +845,6 @@ void tb_flush(CPUState *cpu)
>          > tcg_ctx.code_gen_buffer_size) {
>          cpu_abort(cpu, "Internal error: code buffer overflow\n");
>      }
> -    tcg_ctx.tb_ctx.nb_tbs = 0;
>
>      CPU_FOREACH(cpu) {
>          int i;
> @@ -853,9 +852,10 @@ void tb_flush(CPUState *cpu)
>          for (i = 0; i < TB_JMP_CACHE_SIZE; ++i) {
>              atomic_set(&cpu->tb_jmp_cache[i], NULL);
>          }
> -        cpu->tb_flushed = true;
> +        atomic_mb_set(&cpu->tb_flushed, true);
>      }
>
> +    tcg_ctx.tb_ctx.nb_tbs = 0;
>      qht_reset_size(&tcg_ctx.tb_ctx.htable, CODE_GEN_HTABLE_SIZE);

I can see the sense of moving the setting of nb_tbs but is it strictly
required as part of this patch?

>      page_flush_tb();

Otherwise:

Reviewed-by: Alex Bennée <address@hidden>

--
Alex Bennée