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