Re: [RFC PATCH] accel/tcg: avoid re-translating one-shot instructions

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

Peter Maydell <peter.maydell@linaro.org> writes:

> On Thu, 15 Apr 2021 at 17:25, Alex Bennée <alex.bennee@linaro.org> wrote:
>>
>> By definition a single instruction is capable of being an IO
>> instruction. This avoids a problem of triggering a cpu_io_recompile on
>> a non-recorded translation which then fails because it expects
>> tcg_tb_lookup() to succeed unconditionally. The normal use case
>> requires a TB to be able to resolve machine state.
>>
>> The other users of tcg_tb_lookup() are able to tolerate a missing TB
>> if the machine state has been resolved by other means - which in the
>> single-shot case is always true because machine state is synced at the
>> start of a block.
>>
>> Reported-by: Peter Maydell <peter.maydell@linaro.org>
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> ---
>>  accel/tcg/translate-all.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
>> index ba6ab09790..b12d0898d0 100644
>> --- a/accel/tcg/translate-all.c
>> +++ b/accel/tcg/translate-all.c
>> @@ -1863,7 +1863,7 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
>>
>>      if (phys_pc == -1) {
>>          /* Generate a one-shot TB with 1 insn in it */
>> -        cflags = (cflags & ~CF_COUNT_MASK) | 1;
>> +        cflags = (cflags & ~CF_COUNT_MASK) | CF_LAST_IO | 1;
>>      }
>>
>>      max_insns = cflags & CF_COUNT_MASK;
>> --
>
> Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

Are you going to apply this directly or do you want it through a tree?

>
> thanks
> -- PMM


-- 
Alex Bennée