Re: [PATCH] virtio: use smp_XX barriers

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

On Wed, Jan 27, 2010 at 11:46:32AM +0200, Avi Kivity wrote:
> On 01/27/2010 11:36 AM, Michael S. Tsirkin wrote:
>>
>>
>>> 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.

Intel system programming says:
7.2.3.2 Neither Loads Nor Stores Are Reordered with Like Operations
Doesn't this mean that reads are not reordered?  If no what does it
mean?

>> 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.