Re: [Qemu-devel] [PATCH] [RFC] Add machine type pc-1.0-qemu-kvm for live migrate compatibility with qemu-kvm

[Serge Hallyn <serge.hallyn@ubuntu.com>]

Quoting Michael S. Tsirkin (mst@redhat.com):
> On Tue, Jul 29, 2014 at 08:31:28AM +0100, Alex Bligh wrote:
> > Serge,
> > 
> > > I don't think that is in any way a problem.  Is migrating to older
> > > versions ever actually expected to work?  In either case I don't
> > > think for this particular case it's a problem.
> > 
> > Good; no; and good - respectively.
> > 
> > > (The "how to handle this in libvirt" question is more interesting)
> > 
> > I've been giving this some thought. However rococo we make this,
> > I think the caller is often going to need to modify the command
> > line anyway, i.e. is going to need to be aware of the migration
> > problem.
> > 
> > For instance, taking Ubuntu as an example, 12.04 shipped with
> > qemu-kvm-1.0, and a pxe-virtio.rom of just under 64k, giving
> > a 64k ROM slot. 14.04 ships with qemu-2.0 and a pxe-virtio.rom
> > of over 80k, giving a 128k ROM slot. So however we fix the
> > machine types, when migrating in a 12.04 initiated VM, qemu
> > will need
> >  -global virtio-net-pci.romfile=/path/to/pxe-virtio.rom.12.04
> > on the command line (or, if you don't much care about PXE
> > working on a soft reboot, a blank file of the same size),
> > whereas when migrating in a 14.04 initiated VM, that must
> > not be on the command line.
> > 
> > Fixing this properly would entail requiring that the ROMs are
> > (effectively) distributed with qemu or at least that all
> > ROM sizes become part of the machine type standard. This
> > would have the advantage that loading a larger ROM than
> > the machine type specifies would fail unless the ROM
> > size was explicitly configured on the command line. But
> > this is a subject wider than this patch.
> > 
> > So the long and the short of it is that libvirt (sadly) like
> > anything else starting qemu machines needs to know a bit about
> > different versions of qemu, and be able to replace a machine
> > type option with a machine type option and more on the
> > command line.
> > 
> > My previous suggestion doesn't help much because qemu will
> > still need to be passed something on the command line.
> > 
> > I think the best way to go with this patch would be something
> > like:
> > 
> > * Add pc-1.0-qemu-kvm as a machine type (done)
> > 
> > * Rename pc-1.0 to pc-1.0-qemu-git
> > 
> > * Add an alias for pc-1.0 to either pc-1.0-qemu-git or
> >   pc-1.0-qemu-kvm, configurable at build time with
> >   a ./configure option. The distro can then set this
> >   appropriately. This would default to pc-1.0-qemu-git
> >   (i.e. the current behaviour).
> > 
> > On distros that only every used one of the above,
> > ./configure will sort things out, +/- self-inflicted
> > injuries relating to ROM size changes. If life is
> > more complicated, libvirt (and other callers) will
> > need to be aware. pc-1.0-qemu-git and pc-1.0-qemu-kvm
> > can be used to unambiguously mean the relevant machine
> > type, which will fix things going forward for that
> > machine type.
> > 
> > WDYT?
> 
> 
> This just means we perpetuate the broken-ness.
> 
> I would rather we teach libvirt to do the right thing
> unconditionally.

Well, now, here's a thought - can we hot-patch qemu to
change the machine type while it is running before the
migrate?  Just s/pc-1.0/pc-x.0/ or something?