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

From: Rusty Russell
Subject: Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself
Date: Fri, 7 May 2010 12:53:52 +0930
User-agent: KMail/1.13.2 (Linux/2.6.32-21-generic; KDE/4.4.2; i686; ; )

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.

Hope that clarifies,

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