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

On Mon, Aug 04, 2014 at 03:08:31PM +0000, Serge Hallyn wrote: > Quoting Michael S. Tsirkin (mst(a)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? Frankly I don't see what will this accomplish. If you really want it to be called pc-1.0, you can make it a machine property instead. E.g. qemu-kvm-compatibility. Teach management to set it if remote is qemu-kvm: -machine pc-1.0,qemu-kvm-compatibility=on