On Tue, 2014-11-11 at 17:57 +0200, Michael S. Tsirkin wrote:
> On Tue, Nov 11, 2014 at 12:17:26PM +0000, Ben Hutchings wrote:
> > On Tue, 2014-11-11 at 10:58 +0000, Stefan Hajnoczi wrote:
> > > Commit 3d0ad09412ffe00c9afa201d01effdb6023d09b4 ("drivers/net: Disable
> > > UFO through virtio") breaks live migration of KVM guests from older to
> > > newer host kernels:
> > >
> > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=3d0ad09412ffe00c9afa201d01effdb6023d09b4
> > >
> > > The problem occurs when a guest running on a host kernel without commit
> > > 3d0ad0941 in tun.ko attempts to live migration to a host with commit
> > > 3d0ad0941.
> > >
> > > Live migration fails in QEMU with the following error message:
> > >
> > > virtio-net: saved image requires TUN_F_UFO support
> > >
> > > The old host tun.ko advertised support for TUN_F_UFO. The new host
> > > tun.ko does not and that's why QEMU aborts live migration. QEMU cannot
> > > change the features of a running virtio-net device.
> >
> > Yes, this is known and was mentioned in the DSA.
> >
> > > tuxcrafter provided logs from two Debian hosts migrating from
> > > 3.2.60-1+deb7u3 to 3.2.63-2+deb7u1:
> > >
> > > http://paste.debian.net/131264/
> > >
> > > I haven't investigated enough to suggest a fix, just wanted to bring it
> > > to your attention. Soon a lot of people will be hitting this problem as
> > > they upgrade their infrastructure and migrate guests - seems like a
> > > critical issue.
> >
> > You can work around this by making macvtap and tun still claim to
> > support UFO.
>
> If this is what we want userspace to do, let's just put the
> feature flag back?
Let's not *just* put the feature flag back. I accept this is probably
needed as a workaround, but UFO/IPv6 still won't work correctly over
virtio.
> Basically userspace assumed that features will only
> ever be added, never removed, so this change is
> breaking it.
OK.
> > They continue to support it even if it's not advertised
> > because the tap features don't reliably get propagated to virtio
> > devices.
> >
> > Ben.
>
> Hmm I don't understand this last sentence.
> features are actually reliably propagated to virtio devices.
They might be when using current QEMU and libvirt on the host. They
weren't when I tested on Debian stable. The warnings about 'using
disabled UFO feature' are reliably triggered if the host's tap driver is
patched and the guest's virtio_net driver is not.
Ben.
--
Ben Hutchings
Experience is directly proportional to the value of equipment destroyed.
- Carolyn Scheppner
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