linux-kernel - Re: [PATCH 2/5] virtio: support unlocked queue kick

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Date:	Tue, 1 Nov 2011 16:40:45 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Jens Axboe <axboe@...nel.dk>,
	Stefan Hajnoczi <stefanha@...ux.vnet.ibm.com>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] virtio: support unlocked queue kick

On Thu, Oct 06, 2011 at 11:18:28AM -0200, Michael S. Tsirkin wrote:
> On Thu, Oct 06, 2011 at 12:15:36PM +1030, Rusty Russell wrote:
> > On Wed, 05 Oct 2011 15:54:05 -0400, Christoph Hellwig <hch@...radead.org> wrote:
> > > Split virtqueue_kick to be able to do the actual notification outside the
> > > lock protecting the virtqueue.  This patch was originally done by
> > > Stefan Hajnoczi, but I can't find the original one anymore and had to
> > > recreated it from memory.  Pointers to the original or corrections for
> > > the commit message are welcome.
> > 
> > An alternative to this is to update the ring on every virtqueue_add_buf.
> > MST discussed this for virtio_net, and I think it's better in general.
> > 
> > The only reason that it wasn't written that way originally is that the
> > barriers make add_buf slightly more expensive.
> > 
> > Cheers,
> > Rusty.
> 
> With event index, I'm not sure that's enough to make the kick lockless
> anymore.

Hmm, any comment on this? These have been benchmarked
to give a sizeable speedup, so I'm thinking it's better to take
the patches as is, if someone has the inclination to redo
the work with an atomic virtqueue_add_buf, that can
be applied on top.

> 
> -- 
> MST
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