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

[Peter Maydell <peter.maydell@linaro.org>]

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>

thanks
-- PMM