Re: [RFC PATCH] tcg/cpu-exec: precise single-stepping after an exception

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

On Thu, 16 Jul 2020 at 11:08, Luc Michel <luc.michel@greensocs.com> wrote:
>
> When single-stepping with a debugger attached to QEMU, and when an
> exception is raised, the debugger misses the first instruction after the
> exception:

This is a long-standing bug; thanks for looking at it.
(https://bugs.launchpad.net/qemu/+bug/757702)


> diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
> index d95c4848a4..e85fab5d40 100644
> --- a/accel/tcg/cpu-exec.c
> +++ b/accel/tcg/cpu-exec.c
> @@ -502,10 +502,21 @@ static inline bool cpu_handle_exception(CPUState *cpu, int *ret)
>              CPUClass *cc = CPU_GET_CLASS(cpu);
>              qemu_mutex_lock_iothread();
>              cc->do_interrupt(cpu);
>              qemu_mutex_unlock_iothread();
>              cpu->exception_index = -1;
> +
> +            if (unlikely(cpu->singlestep_enabled)) {
> +                /*
> +                 * After processing the exception, ensure an EXCP_DEBUG is
> +                 * raised when single-stepping so that GDB doesn't miss the
> +                 * next instruction.
> +                 */
> +                cpu->exception_index = EXCP_DEBUG;
> +                return cpu_handle_exception(cpu, ret);
> +            }

I like the idea of being able to do this generically in
the main loop.

How about interrupts? If we are single-stepping and we
take an interrupt I guess we want to stop before the first
insn of the interrupt handler rather than after it, which
would imply a similar change to cpu_handle_interrupt().

thanks
-- PMM