Re: [PATCH] Arm: relax barrier in sync_vcpu_execstate()

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 04/02/2026 13:15, Jan Beulich wrote:
> The counterpart in vcpu_context_saved() is smp_wmb(), and here we don't
> really need any more than a read barrier: The concern expressed in the
> comment is void, as updates to the context are necessarily observed ahead
> of ->is_running going false, by virtue of said barrier in
> vcpu_context_saved().
>
> Fixes: f6790389613c ("xen/arm: sched: Ensure the vCPU context is seen before 
> vcpu_pause() returns")
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Acked-by: Julien Grall <julien@xxxxxxx>

> ---
> As to the tag, this is one of the cases where I'm on the edge between
> Amends: and Fixes:.
Good question. I would not consider the patch as backport and it is 
border line a bug fix (the barrier is just too strong) so Amends may be 
better. I will leave it up to you when you commit.
> Subsequently we may want to move the barrier into the sole common code
> caller, seeing that the other barrier also is in common code. Furthermore,
> seeing that for all but x86 the function is then entirely empty, we may
> want to allow it to be inline to avoid the unnecessary (tail) call.
+1 for both proposal.

> --- a/xen/arch/arm/domain.c
> +++ b/xen/arch/arm/domain.c
> @@ -342,11 +342,8 @@ void sync_vcpu_execstate(struct vcpu *v)
>        * Per vcpu_context_saved(), the context can be observed when
>        * v->is_running is false (the caller should check it before calling
>        * this function).
> -     *
> -     * Note this is a full barrier to also prevent update of the context
> -     * to happen before it was observed.
>        */
> -    smp_mb();
> +    smp_rmb();
>   }
>   
>   #define NEXT_ARG(fmt, args)                                                 \
Cheers,

--
Julien Grall