Re: [PATCH] accel/tcg: mttcg remove false-negative halted assertion

[Richard Henderson]

On 8/28/23 18:06, Nicholas Piggin wrote:
> mttcg asserts that an execution ending with EXCP_HALTED must have
> cpu->halted. However between the event or instruction that sets
> cpu->halted and requests exit and the assertion here, an
> asynchronous event could clear cpu->halted.
>
> This leads to crashes running AIX on ppc/pseries because it uses
> H_CEDE/H_PROD hcalls, where H_CEDE sets self->halted = 1 and
> H_PROD sets other cpu->halted = 0 and kicks it.
>
> H_PROD could be turned into an interrupt to wake, but several other
> places in ppc, sparc, and semihosting follow what looks like a similar
> pattern setting halted = 0 directly. So remove this assertion.
>
> Reported-by: Ivan Warren <ivan@vmfacility.fr>
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>   accel/tcg/tcg-accel-ops-mttcg.c | 11 -----------
>   1 file changed, 11 deletions(-)

The adjustments of 'halted' and 'prod' are done under the io lock in both cases, so 
there's no race there.

It is perfectly reasonable that after thread A sets halted and drops the lock, thread B 
may acquire the lock and clear halted before thread A has a chance to complete longjmp and 
cycle through its main loop.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


> diff --git a/accel/tcg/tcg-accel-ops-mttcg.c b/accel/tcg/tcg-accel-ops-mttcg.c
> index b276262007..d0b6f288d9 100644
> --- a/accel/tcg/tcg-accel-ops-mttcg.c
> +++ b/accel/tcg/tcg-accel-ops-mttcg.c
> @@ -98,17 +98,6 @@ static void *mttcg_cpu_thread_fn(void *arg)
>               case EXCP_DEBUG:
>                   cpu_handle_guest_debug(cpu);
>                   break;
> -            case EXCP_HALTED:
> -                /*
> -                 * during start-up the vCPU is reset and the thread is
> -                 * kicked several times. If we don't ensure we go back
> -                 * to sleep in the halted state we won't cleanly
> -                 * start-up when the vCPU is enabled.
> -                 *
> -                 * cpu->halted should ensure we sleep in wait_io_event
> -                 */
> -                g_assert(cpu->halted);
> -                break;

I adjusted the patch to keep the case label and update the comment, still 
dropping the assert.

Queued to tcg-next.


r~