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    SubjectRe: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself
    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


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