Re: [PATCH v2 06/10] accel/tcg: Use CF_NO_GOTO_{TB, PTR} in cpu_exec_step_atomic

[Peter Maydell]

On Mon, 12 Jul 2021 at 16:46, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> Request that the one TB returns immediately, so that
> we release the exclusive lock as soon as possible.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  accel/tcg/cpu-exec.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
> index 2206c463f5..5bb099174f 100644
> --- a/accel/tcg/cpu-exec.c
> +++ b/accel/tcg/cpu-exec.c
> @@ -330,8 +330,7 @@ void cpu_exec_step_atomic(CPUState *cpu)
>      CPUArchState *env = (CPUArchState *)cpu->env_ptr;
>      TranslationBlock *tb;
>      target_ulong cs_base, pc;
> -    uint32_t flags;
> -    uint32_t cflags = (curr_cflags(cpu) & ~CF_PARALLEL) | 1;
> +    uint32_t flags, cflags;
>      int tb_exit;
>
>      if (sigsetjmp(cpu->jmp_env, 0) == 0) {
> @@ -341,8 +340,14 @@ void cpu_exec_step_atomic(CPUState *cpu)
>          cpu->running = true;
>
>          cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
> -        tb = tb_lookup(cpu, pc, cs_base, flags, cflags);
>
> +        cflags = curr_cflags(cpu);
> +        /* Execute in a serial context. */
> +        cflags &= ~CF_PARALLEL;
> +        /* After 1 insn, return and release the exclusive lock. */
> +        cflags |= CF_NO_GOTO_TB | CF_NO_GOTO_PTR | 1;
> +
> +        tb = tb_lookup(cpu, pc, cs_base, flags, cflags);
>          if (tb == NULL) {
>              mmap_lock();
>              tb = tb_gen_code(cpu, pc, cs_base, flags, cflags);

So previously we would have executed possibly a chain of TBs
before releasing the lock, and now we definitely execute just one?
(I guess the execute-a-chain case is unlikely given the TB
only has one insn and we know it's an exclusive insn...)

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

thanks
-- PMM