linux-kernel - Re: [PATCHv2-RFC 0/2] virtio: put last seen used index into ring itself

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Message-ID: <20100531122055.GB1704@redhat.com>
Date:	Mon, 31 May 2010 15:20:56 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	qemu-devel@...gnu.org
Subject: Re: [PATCHv2-RFC 0/2] virtio: put last seen used index into ring
	itself

On Mon, May 31, 2010 at 05:16:42PM +0930, Rusty Russell wrote:
> On Thu, 27 May 2010 05:20:35 am Michael S. Tsirkin wrote:
> > Here's a rewrite of the original patch with a new layout.
> > I haven't tested it yet so no idea how this performs, but
> > I think this addresses the cache bounce issue raised by Avi.
> > Posting for early flames/comments.
> 
> Sorry, not without some evidence that it'll actually reduce cacheline
> bouncing.  I *think* it will, but it's not obvious: the host may keep
> looking at avail_idx as we're updating last_seen.  Or does qemu always
> look at both together anyway?
> Can someone convince me this is a win?
> Rusty.

What really happens is host looks at flags and last_seen together.
And flags happens to be in the same cache line with avail idx.
So to get an obvious win, we should put flags and last_seen
in a separate cache line from avail, which us easy - just add some padding.

And I'll relax the requirement from guest to only require it to update
last_seen when interrupts are enabled. This way flags and
last_seen are written together and read together.

Makes sense?
-- 
MST
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