Re: [PATCH v2 1/2] xen/events: access last_priority and last_vcpu_id together

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.10.2020 11:27, Juergen Gross wrote:
> The queue for a fifo event is depending on the vcpu_id and the
> priority of the event. When sending an event it might happen the
> event needs to change queues and the old queue needs to be kept for
> keeping the links between queue elements intact. For this purpose
> the event channel contains last_priority and last_vcpu_id values
> elements for being able to identify the old queue.
> 
> In order to avoid races always access last_priority and last_vcpu_id
> with a single atomic operation avoiding any inconsistencies.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

I seem to vaguely recall that at the time this seemingly racy
access was done on purpose by David. Did you go look at the
old commits to understand whether there really is a race which
can't be tolerated within the spec?

> --- a/xen/include/xen/sched.h
> +++ b/xen/include/xen/sched.h
> @@ -114,8 +114,7 @@ struct evtchn
>          u16 virq;      /* state == ECS_VIRQ */
>      } u;
>      u8 priority;
> -    u8 last_priority;
> -    u16 last_vcpu_id;
> +    u32 fifo_lastq;    /* Data for fifo events identifying last queue. */

This grows struct evtchn's size on at least 32-bit Arm. I'd
like to suggest including "priority" in the union, and call the
new field simply "fifo" or some such.

Jan