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

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Message-Id: <201005071253.53393.rusty@rustcorp.com.au>
Date:	Fri, 7 May 2010 12:53:52 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Avi Kivity <avi@...hat.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	qemu-devel@...gnu.org
Subject: Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into 
 ring	itself

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,
Rusty.
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