linux-kernel - Re: [PATCHv3 1/2] virtio: support layout with avail ring before idx

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Message-Id: <201006042046.49872.rusty@rustcorp.com.au>
Date:	Fri, 4 Jun 2010 20:46:49 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	qemu-devel@...gnu.org, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCHv3 1/2] virtio: support layout with avail ring before idx

On Fri, 4 Jun 2010 08:05:43 pm Michael S. Tsirkin wrote:
> On Fri, Jun 04, 2010 at 12:04:57PM +0930, Rusty Russell wrote:
> > On Wed, 2 Jun 2010 12:17:12 am Michael S. Tsirkin wrote:
> > > This adds an (unused) option to put available ring before control (avail
> > > index, flags), and adds padding between index and flags. This avoids
> > > cache line sharing between control and ring, and also makes it possible
> > > to extend avail control without incurring extra cache misses.
> > > 
> > > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > 
> > No no no no.  254?  You're trying to Morton me![1]
> 
> Hmm, I wonder what will we do if we want a 3rd field on
> a separate chacheline. But ok.
> 
> > How's this (untested):
> 
> I think we also want to put flags there as well,
> they are used on interrupt path, together with last used index.

I'm uncomfortable with moving a field.

We haven't done that before and I wonder what will break with old code.

Should we instead just abandon the flags field and use last_used only?
Or, more radically, put flags == last_used when the feature is on?

Thoughts?
Rusty.
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