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

[Sergey Fedorov <serge.fdrv@gmail.com>]

On 14/07/16 15:45, Alex Bennée wrote:
> Sergey Fedorov <sergey.fedorov@linaro.org> writes:
>
>> From: Sergey Fedorov <serge.fdrv@gmail.com>
>>
>> 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.

Do you mean atomic_set() vs. atomic_mb_set()?

>
>> In future, before enabling MTTCG in system mode, tb_flush() must be safe
>> and this field becomes unnecessary.
>>
>> Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
>> Signed-off-by: Sergey Fedorov <sergey.fedorov@linaro.org>
>> ---
>>  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?

Yes, otherwise tb_alloc() may start allocation TBs from the beginning of
the translation buffer before 'tb_flushed' is updated.

Kind regards,
Sergey

>
>>      page_flush_tb();
> Otherwise:
>
> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
>
> --
> Alex Bennée