On Wed, Jan 27, 2010 at 11:28:32AM +0200, Avi Kivity wrote: > On 01/21/2010 07:10 PM, Michael S. Tsirkin wrote: >> Documentation/memory-barriers.txt says: >> Mandatory barriers should not be used to control SMP effects, since >> mandatory barriers unnecessarily impose overhead on UP systems. >> >> This rule applies to virtio, so let's do it correctly. >> >> > > This is wrong. A UP guest still runs in parallel with the virtio device.
Yes, you are right here. I forgot that these macros are compiled out on UP guest. Rusty please ignore this patch. > smp_mb() is used for processor-vs-processor ordering, which can't happen > on UP systems, but for process-vs-device, we must use mb(). > > (this shows up if running a UP guest on an SMP host). Currently, yes. But virtio is not a real device. Here's what I was really trying to improve: rmb() is an lfence on x86_64, but smp_rmb() is a barrier() and this is really sufficient for virtio because x86_64 does not reorder memory reads. Does this mean such an optimization would need a new macro? -- MST _______________________________________________ Virtualization mailing list Virtualization@lists.linux-foundation.org https://lists.linux-foundation.org/mailman/listinfo/virtualization
