On Tue, May 11, 2010 at 10:27:22PM +0300, 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? > > Having things on the same cacheline is only worthwhile if they are > accessed at the same time.
Yes, this is what I was trying to say. avail flags and used index *are* accessed at the same time, so there could be an advantage to sharing a cache line there. All this should be kept in mind if we ever do VIRTIO_RING_F_NEW_LAYOUT. -- MST -- 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
