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

[Avi Kivity]

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?

Having things on the same cacheline is only worthwhile if they are 
accessed at the same time.

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