Re: [PATCH] virtio: use smp_XX barriers

["Michael S. Tsirkin" <mst@redhat.com>]

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