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

[Rusty Russell]

On Wed, 12 May 2010 04:57:22 am Avi Kivity wrote:
> On 05/07/2010 06:23 AM, Rusty Russell wrote:
> > On Thu, 6 May 2010 07:30:00 pm Avi Kivity wrote:
> >    
> >> On 05/05/2010 11:58 PM, Michael S. Tsirkin wrote:
> >>      
> >>> + /* We publish the last-seen used index at the end of the available ring.
> >>> +  * It is at the end for backwards compatibility. */
> >>> + vr->last_used_idx =&(vr)->avail->ring[num];
> >>> + /* Verify that last used index does not spill over the used ring. */
> >>> + BUG_ON((void *)vr->last_used_idx +
> >>> +        sizeof *vr->last_used_idx>   (void *)vr->used);
> >>>    }
> >>>
> >>>        
> >> Shouldn't this be on its own cache line?
> >>      
> > It's next to the available ring; because that's where the guest publishes
> > its data.  That whole page is guest-write, host-read.
> >
> > Putting it on a cacheline by itself would be a slight pessimization; the 
> > host
> > cpu would have to get the last_used_idx cacheline and the avail descriptor
> > cacheline every time.  This way, they are sometimes the same cacheline.
> 
> If one peer writes the tail of the available ring, while the other reads 
> last_used_idx, it's a false bounce, no?

I think we're talking about the last 2 entries of the avail ring.  That means
the worst case is 1 false bounce every time around the ring.  I think that's
why we're debating it instead of measuring it :)

Note that this is a exclusive->shared->exclusive bounce only, too.

Cheers,
Rusty.