netdev - Re: [RFC PATCH 0/4] Implement multiqueue virtio-net

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Date:	Thu, 9 Sep 2010 12:40:27 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Krishna Kumar2 <krkumar2@...ibm.com>
Cc:	anthony@...emonkey.ws, Avi Kivity <avi@...hat.com>,
	davem@...emloft.net, kvm@...r.kernel.org, mst@...hat.com,
	netdev@...r.kernel.org, rusty@...tcorp.com.au
Subject: Re: [RFC PATCH 0/4] Implement multiqueue virtio-net

On Wednesday 08 September 2010, Krishna Kumar2 wrote:
> > On Wednesday 08 September 2010, Krishna Kumar2 wrote:
> > > > The new guest and qemu code work with old vhost-net, just with
> reduced
> > > > performance, yes?
> > >
> > > Yes, I have tested new guest/qemu with old vhost but using
> > > #numtxqs=1 (or not passing any arguments at all to qemu to
> > > enable MQ). Giving numtxqs > 1 fails with ENOBUFS in vhost,
> > > since vhost_net_set_backend in the unmodified vhost checks
> > > for boundary overflow.
> > >
> > > I have also tested running an unmodified guest with new
> > > vhost/qemu, but qemu should not specify numtxqs>1.
> >
> > Can you live migrate a new guest from new-qemu/new-kernel
> > to old-qemu/old-kernel, new-qemu/old-kernel and old-qemu/new-kernel?
> > If not, do we need to support all those cases?
> 
> I have not tried this, though I added some minimal code in
> virtio_net_load and virtio_net_save. I don't know what needs
> to be done exactly at this time. I forgot to put this in the
> "Next steps" list of things to do.

I was mostly trying to find out if you think it should work
or if there are specific reasons why it would not.
E.g. when migrating to a machine that has an old qemu, the guest
gets reduced to a single queue, but it's not clear to me how
it can learn about this, or if it can get hidden by the outbound
qemu.

	Arnd
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