Re: [Xen-devel] [PATCH 8/8] evtchn: add FIFO-based event channel hypercalls and port ops

[Tim Deegan <tim@xxxxxxx>]

At 13:55 +0000 on 20 Mar (1363787704), Jan Beulich wrote:
> >>> On 20.03.13 at 14:42, David Vrabel <david.vrabel@xxxxxxxxxx> wrote:
> > On 20/03/13 10:47, Jan Beulich wrote:
> >>>>> On 19.03.13 at 22:00, David Vrabel <david.vrabel@xxxxxxxxxx> wrote:
> >>> +struct evtchn_fifo_queue {
> >>> +    volatile uint32_t *head; /* points into control block */
> >> 
> >> I still think that explicit barriers are the way to go. Unless Linux'es
> >> view on this has changed, you'll have issues getting the Linux folks
> >> to accept this.
> > 
> > This volatile can just be removed.  head is only written by Xen in one
> > place and it is immediately followed by a spin_unlock() which is a barrier.
> 
> A release type barrier only, but that presumably is sufficient for
> the purposes here.

You might have to use an atomic_t or similar if the consumer might be
confused by partial updates.

Tim.

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