Re: [PATCH] accel/tcg/cpu-exec: fix precise single-stepping after interrupt

[Richard Henderson <richard.henderson@linaro.org>]

On 2/14/22 03:26, Luc Michel wrote:
> In some cases, cpu->exit_request can be false after handling the
> interrupt, leading to another TB being executed instead of returning
> to the main loop.
> 
> Fix this by returning true unconditionally when in single-step mode.
> 
> Fixes: ba3c35d9c4026361fd380b269dc6def9510b7166
> 
> Signed-off-by: Luc Michel <lmichel@kalray.eu>
> ---
> Coming back on this issue I worked on with Richard in 2020. The issue is
> that when debugging the guest with GDB, the first instruction of the IRQ
> handler is missed by GDB (it's still executed though).
> 
> It happened to me again in TCG RR mode (but not in MTTCG). It seems that
> cpu->exit_request can be false in RR mode when returning from
> cc->tcg_ops->cpu_exec_interrupt, leading to cpu_handle_interrupt
> returning false and the next TB being executed, instead of the EXCP_DEBUG
> being handled.
> ---
>   accel/tcg/cpu-exec.c | 10 +++++++---
>   1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
> index 8b4cd6c59d..74d7f83f34 100644
> --- a/accel/tcg/cpu-exec.c
> +++ b/accel/tcg/cpu-exec.c
> @@ -796,13 +796,17 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
>                   /*
>                    * After processing the interrupt, ensure an EXCP_DEBUG is
>                    * raised when single-stepping so that GDB doesn't miss the
>                    * next instruction.
>                    */
> -                cpu->exception_index =
> -                    (cpu->singlestep_enabled ? EXCP_DEBUG : -1);
> -                *last_tb = NULL;
> +                if (unlikely(cpu->singlestep_enabled)) {
> +                    cpu->exception_index = EXCP_DEBUG;
> +                    return true;

By returning here, you also need to qemu_mutex_unlock_iothread().

> +                } else {

You can remove the else after the return.
Otherwise this looks good; sorry for the delay.


r~