Re: [PATCH] virtio: use smp_XX barriers

On 01/27/2010 11:36 AM, Michael S. Tsirkin wrote:

quoted

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.

   

x86_64 can do speculative and reordered reads.

Does this mean such an optimization would need a new macro?
   

Please no.

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