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Date: Wed, 24 Feb 2010 07:29:25 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [Qemu-devel] Re: [PATCH-RFC 13/13] virtio-net: connect to
	vhost net backend
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	<201002240314.25631.paul@codesourcery.com>
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To: Paul Brook <paul@codesourcery.com>
Cc: qemu-devel@nongnu.org

On Wed, Feb 24, 2010 at 03:14:25AM +0000, Paul Brook wrote:
> > vnet_hdr is IMHO a really bad example to copy from.
> > 
> > vnet_hdr leaks into the migration state via n->has_vnet_hdr.  What this
> > means is that if you want to migrate from -net tap -net nic,model=virtio
> > to -net user -net nic,model=virtio, it will fail.
> > 
> > This is a hard problem to solve in qemu though because it would require
> > that we implement software GSO which so far, no one has stepped up to do.
> 
> Or make virtio-net pass this on to the guest, and have that deal with the 
> problem.

This is exacly what we do, via feature bits.

> If you implement software GSO, then devices can assume it's always 
> present and don't need to probe.
> 
> Paul