On Wed, 12 May 2010 04:57:22 am Avi Kivity wrote: > On 05/07/2010 06:23 AM, Rusty Russell wrote: > > On Thu, 6 May 2010 07:30:00 pm Avi Kivity wrote: > > > >> On 05/05/2010 11:58 PM, Michael S. Tsirkin wrote: > >> > >>> + /* We publish the last-seen used index at the end of the available ring. > >>> + * It is at the end for backwards compatibility. */ > >>> + vr->last_used_idx =&(vr)->avail->ring[num]; > >>> + /* Verify that last used index does not spill over the used ring. */ > >>> + BUG_ON((void *)vr->last_used_idx + > >>> + sizeof *vr->last_used_idx> (void *)vr->used); > >>> } > >>> > >>> > >> Shouldn't this be on its own cache line? > >> > > It's next to the available ring; because that's where the guest publishes > > its data. That whole page is guest-write, host-read. > > > > Putting it on a cacheline by itself would be a slight pessimization; the > > host > > cpu would have to get the last_used_idx cacheline and the avail descriptor > > cacheline every time. This way, they are sometimes the same cacheline. > > If one peer writes the tail of the available ring, while the other reads > last_used_idx, it's a false bounce, no?
I think we're talking about the last 2 entries of the avail ring. That means the worst case is 1 false bounce every time around the ring. I think that's why we're debating it instead of measuring it :) Note that this is a exclusive->shared->exclusive bounce only, too. Cheers, Rusty. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
