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Date: Wed, 16 Sep 2015 17:55:11 +0200
From: Greg Kurz <gkurz@linux.vnet.ibm.com>
Message-ID: <20150916175511.6328fceb@bahia.local>
In-Reply-To: <20150911103021.19f73181.cornelia.huck@de.ibm.com>
References: <1441958516-21107-1-git-send-email-jasowang@redhat.com>
	<20150911103021.19f73181.cornelia.huck@de.ibm.com>
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Subject: Re: [Qemu-devel] [PATCH] virtio-net: unbreak self announcement and
 guest offloads after migration
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To: Cornelia Huck <cornelia.huck@de.ibm.com>
Cc: qemu-devel@nongnu.org, Jason Wang <jasowang@redhat.com>, mst@redhat.com, qemu-stable@nongnu.org, Gerd Hoffmann <kraxel@redhat.com>

On Fri, 11 Sep 2015 10:30:21 +0200
Cornelia Huck <cornelia.huck@de.ibm.com> wrote:

> On Fri, 11 Sep 2015 16:01:56 +0800
> Jason Wang <jasowang@redhat.com> wrote:
> 
> > After commit 019a3edbb25f1571e876f8af1ce4c55412939e5d ("virtio: make
> > features 64bit wide"). Device's guest_features was actually set after
> > vdc->load(). This breaks the assumption that device specific load()
> > function can check guest_features. For virtio-net, self announcement
> > and guest offloads won't work after migration.
> > 
> > Fixing this by defer them to virtio_net_load() where guest_features
> > were guaranteed to be set. Other virtio devices looks fine.
> > 
> > Fixes: 019a3edbb25f1571e876f8af1ce4c55412939e5d
> >        ("virtio: make features 64bit wide")
> > Cc: qemu-stable@nongnu.org
> > Cc: Gerd Hoffmann <kraxel@redhat.com>
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > ---
> >  hw/net/virtio-net.c | 40 +++++++++++++++++++++++-----------------
> >  1 file changed, 23 insertions(+), 17 deletions(-)
> 
> Migration support for virtio is really a twisty maze, it's easy to make
> mistakes like that :(
> 

We have the very same problem with @device_endian which is also streamed in
a subsection. To prevent early usage on the load path, we set @device_endian
to a poisoned value that triggers assert() in the virtio_is_big_endian() helper.

Should this logic be generalized ?