On Wed, Jun 17, 2015 at 02:20:30PM +0200, Paolo Bonzini wrote: > > > On 17/06/2015 13:58, Michael S. Tsirkin wrote: > > > No, it doesn't. The guest misbehaves maybe, but the migration format is > > > not affected. > > > > I just tried, set prog_if to different values, sure it failed. > > How so? It's just a byte in config space. But even then, fixing > migration is just a side effect of keeping config space consistent for a > given machine type (i.e. not changing hardware type under the guest's feet).
David's patches are also guest visible, are they not? We are losing state guest can indirectly observe, right? > > Here's another one, at random: > > > > Author: Michael S. Tsirkin <m...@redhat.com> > > Date: Thu Feb 14 19:11:27 2013 +0200 > > > > e1000: unbreak the guest network migration to 1.3 > > > > QEMU 1.3 does not emulate the link auto negotiation, so if migrate to a > > 1.3 machine during link auto negotiation, the guest link will be set to > > down. > > Fix this by just disabling auto negotiation for 1.3 and older. > > > > Signed-off-by: Michael S. Tsirkin <m...@redhat.com> > > Okay, that's an interesting one, and there's a similar one for e1000 > interrupt mitigation. > > The interesting point is that in both cases the bug compatibility > extends to other behavior of the device, i.e. more than just migration. But the guest registers are exactly the same. It is only guest-visible indirectly, as timing of link up events. So why not keep auto-negotiation running correctly? Because we can't keep it running across migration. > I would even say that bug-compatibility of migration is just a side > effect, not the primary end. For interrupt mitigation, it was not > enabled on older machine types in the first place, because it could > break guests. Keeping backwards migration working was just a side > effect; simply, checking "s->compat_flags & E1000_FLAG_MIT" is the only > sensible way to write e1000_mit_state_needed. Auto negotiation should > have been done the same way, which is what your patch did. > > Paolo True, David's patches only trigger if migration happens. I am guessing there is no need to touch other paths, but I am not very familiar with the hardware in question. -- MST
