* Michael S. Tsirkin (m...@redhat.com) wrote:
> On Thu, Jun 30, 2016 at 10:34:51AM +0200, Robin Geuze wrote:
> > Hey,
> >
> > I work for TransIP and we host a VPS platform based on QEMU/KVM. We are
> > currently running qemu 2.4.0. A few days ago we noticed that live migrations
> > for some of our VM's would fail. Further investigation turned out it was
> > specific to windows server 2012, caused by the fact that the standard virtio
> > driver from RedHat was replaced in windows updates by a driver called
> > "Midfin eFabric" (this driver doesn't really seem to be meant for virtio, we
> > have a case running at MicroSoft about that). Once we knew how to reproduce
> > we tested this on QEMU 2.6.0 as well and it also seems to be affected
> > (later we found out that 2.4.0 to 2.6.0 migration does work probably due to
> > pure luck).
> >
> > We started investigating the problem in QEMU 2.4.0 and noticed it was caused
> > by the fact that virtio_net_device_load requires certain feature flags to be
> > set, specifically to load curr_guest_offloads which is only written and read
> > if the VIRTIO_NET_F_CTRL_GUEST_OFFLOADS flag is set, but those flags are set
> > in virtio_load after the call to virtio_net_device_load. Moving the code
> > setting the feature flags before the call to virtio_net_device_load fixes
> > it, however it introduces another problem. Virtio can have 64-bits feature
> > flags, however the standard save payload for virtio only has space for
> > 32-bits feature flags. This was solved by putting those in a subsection of
> > the vmstate_save_state stuff. Unfortunately this is called (and thus binary
> > offset located) after the virtio_net_device_load code.
> >
> > There was an attempt to fix this in QEMU 2.6.0. However, this seems to have
> > broken it worse. The write code (virtio_net_save, virtio_save and
> > virtio_net_save_device) still puts the curr_guest_offloads value before the
> > vmstate_save_state data. However the read code expects and tries to read it
> > after the vmstate_save_state data. Should we just also change the
> > virtio_net_save code to have it follow the same order as virtio_net_load? Or
> > will this potentially break more stuff.
> >
> > Regards,
> >
> > Robin Geuze
> >
> > TransIP BV
>
> After going over it several times, I think the change in 2.6
> was wrong
>
>
>
> commit 1f8828ef573c83365b4a87a776daf8bcef1caa21
> Author: Jason Wang <jasow...@redhat.com>
> Date: Fri Sep 11 16:01:56 2015 +0800
>
> virtio-net: unbreak self announcement and guest offloads after migration
>
> 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-sta...@nongnu.org
> Cc: Gerd Hoffmann <kra...@redhat.com>
> Signed-off-by: Jason Wang <jasow...@redhat.com>
> Reviewed-by: Michael S. Tsirkin <m...@redhat.com>
> Signed-off-by: Michael S. Tsirkin <m...@redhat.com>
> Reviewed-by: Cornelia Huck <cornelia.h...@de.ibm.com>
>
>
> I'm not sure what was I thinking when I applied this:
> it changes load without changing save - how can this work?
>
>
> I am inclined to revert 1f8828ef573c83365b4a87a776daf8bcef1caa21 and
> apply this instead:
>
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 7ed06ea..18153d5 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -1499,6 +1499,16 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int
> version_id)
> }
> qemu_get_be32s(f, &features);
>
> + /*
> + * Temporarily set guest_features low bits - needed by
> + * virtio net load code testing for VIRTIO_NET_F_CTRL_GUEST_OFFLOADS
> + * VIRTIO_NET_F_GUEST_ANNOUNCE and VIRTIO_NET_F_CTRL_VQ.
> + *
> + * Note: devices should always test host features in future - don't
> create
> + * new dependencies like this.
> + */
> + vdev->guest_features = features;
> +
> config_len = qemu_get_be32(f);
>
> /*
>
> Could you please confirm whether this help?
> Jason, Cornelia - any comments?
>
> David, if this goes in I'm afraid your patchset reworking
> save/load will have to be rebased, but I think we want
> the bugfix first and new features/changes second.
> Do you agree?
Yes, bug fixes first. But actually the merge is trivial, the only
place I think they collide is in virtio_net_load and it's just the difference
of the return virtio_load vs the ret = virtio_load in the patch you revert.
Dave
> --
> MST
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
