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    SubjectRe: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself
    On Mon, May 24, 2010 at 09:37:05AM +0300, Avi Kivity wrote:
    > On 05/23/2010 07:30 PM, Michael S. Tsirkin wrote:
    >>
    >>
    >>>> Maybe we should use atomics on index then?
    >>>>
    >>>>
    >>> This should only be helpful if you access the cacheline several times in
    >>> a row. That's not the case in virtio (or here).
    >>>
    >> So why does it help?
    >>
    >
    > We actually do access the cacheline several times in a row here (but not
    > in virtio?):
    >
    >> case SHARE:
    >> while (count< MAX_BOUNCES) {
    >> /* Spin waiting for other side to change it. */
    >> while (counter->cacheline1 != count);
    >>
    >
    > Broadcast a read request.
    >
    >> count++;
    >> counter->cacheline1 = count;
    >>
    >
    > Broadcast an invalidate request.
    >
    >> count++;
    >> }
    >> break;
    >>
    >> case LOCKSHARE:
    >> while (count< MAX_BOUNCES) {
    >> /* Spin waiting for other side to change it. */
    >> while (__sync_val_compare_and_swap(&counter->cacheline1, count, count+1)
    >> != count);
    >>
    >
    > Broadcast a 'read for ownership' request.
    >
    >> count += 2;
    >> }
    >> break;
    >>
    >
    > So RMW should certainly by faster using single-instruction RMW
    > operations (or using prefetchw).

    Okay, but why is lockunshare faster than unshare?

    > --
    > Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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