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

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Date:	Mon, 22 Aug 2011 09:54:50 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>,
	Sasha Levin <levinsasha928@...il.com>
Cc:	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 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.
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