Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself

[Avi Kivity]

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.