linux-kernel - Re: [PATCH] virtio-net: Read MAC only after initializing MSI-X

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Date:	Mon, 22 Aug 2011 11:36:13 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Sasha Levin <levinsasha928@...il.com>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH] virtio-net: Read MAC only after initializing MSI-X

On Mon, Aug 22, 2011 at 09:54:50AM +0930, Rusty Russell wrote:
> On Fri, 19 Aug 2011 18:23:35 +0300, "Michael S. Tsirkin" <mst@...hat.com> wrote:
> > On Sat, Aug 13, 2011 at 11:51:01AM +0300, Sasha Levin wrote:
> > > The MAC of a virtio-net device is located at the first field of the device
> > > specific header. This header is located at offset 20 if the device doesn't
> > > support MSI-X or offset 24 if it does.
> > > 
> > > Current code in virtnet_probe() used to probe the MAC before checking for
> > > MSI-X, which means that the read was always made from offset 20 regardless
> > > of whether MSI-X in enabled or not.
> > > 
> > > This patch moves the MAC probe to after the detection of whether MSI-X is
> > > enabled. This way the MAC will be read from offset 24 if the device indeed
> > > supports MSI-X.
> > > 
> > > Cc: Rusty Russell <rusty@...tcorp.com.au>
> > > Cc: Michael S. Tsirkin <mst@...hat.com>
> > > Cc: virtualization@...ts.linux-foundation.org
> > > Cc: netdev@...r.kernel.org
> > > Cc: kvm@...r.kernel.org
> > > Signed-off-by: Sasha Levin <levinsasha928@...il.com>
> > 
> > I am not sure I see a bug in virtio: the config pace layout simply
> > changes as msix is enabled and disabled (and if you look at the latest
> > draft, also on whether 64 bit features are enabled).
> > It doesn't depend on msix capability being present in device.
> > 
> > The spec seems to be explicit enough:
> > 	If MSI-X is enabled for the device, two additional fields immediately
> > 	follow this header.
> > 
> > So I'm guessing the bug is in kvm tools which assume
> > same layout for when msix is enabled and disabled.
> > qemu-kvm seems to do the right thing so the device
> > seems to get the correct mac.
> 
> So, the config space moves once MSI-X is enabled?  In which case, it
> should say "ONCE MSI-X is enabled..."
> 
> Thanks,
> Rusty.

Yes. Or maybe 'WHEN' - since if MSI-X is disabled again, it moves back.
Let's update the spec to make it clearer?

-- 
MST
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