On Fri, Feb 08, 2013 at 04:34:58PM +0100, Borislav Petkov wrote: > On Fri, Feb 08, 2013 at 01:19:02PM -0200, Eduardo Habkost wrote: > > "-machine pc-1.3 -cpu n270" (and older machine-types) needs to keep > > MOVBE disabled, or you will break live migration. > > > > Personally I wouldn't mind declaring n270 as a non-migratable CPU > > model. But libvirt supports n270, so it already expects n270 to expose > > a stable ABI on each machine-type. > > From looking at the code - and I've never looked at qemu code before > last night - we could filter migration features in x86_cpu_realize() > when called down the pc_init1() path.
x86_cpu_realize() is too late, as it would make "-cpu n270,-movbe" fail to disable the feature. The features need to be be initialized properly before cpu_x86_parse_featurestr(), on cpu_x86_register(). > > We simply need to look the QEMUMachine.is_default flag and leave the > feature on only if pc-1.4 which sets the is_default thing. is_default has nothing to do with it. If we enable MOVBE on pc-1.4, pc-1.4 will need to keep MOVBE enabled forever, it doesn't matter if it is the default machine-type or not. > > Or is there a more preferred way to do that? pc_init_pci_1_3() needs to ask the CPU code to disable MOVBE on n270. As we don't have a decent method to do that today, we are using static variables and compatibility-setup functions called from the machine init function. See disable_kvm_pv_eoi() for example. One day we will be able to do that using properties on the machine-type compat_props tables, but we can't do that yet. People can easily work around the lack of the feature today, using "-cpu n270,+movbe", so I believe it is worth waiting a little bit and fixing this on QEMU 1.5, when we will (hopefully) have a decent compatibility system based on compat_props. -- Eduardo
