Re: [libvirt] [PATCH] qemu: Use legacy USB on ppc64

On Tue, Jul 19, 2016 at 07:04:36PM +0200, Andrea Bolognani wrote: >  > On Tue, 2016-07-19 at 16:17 +0200, Andrea Bolognani wrote: > >  > > Commit 8156493d8db9 changed libvirt so that '-device pci-ohci' > > would be used instead of '-usb' on ppc64 when no specific USB > > controller model had been specified in the guest configuration. > >   > > While the device that ends up being presented by the guest is > > exactly the same, '-usb' causes it to be assigned to PCI address > > 00:00.0 while '-device pci-ohci', being subject to the regular > > PCI address assignment logic, will be at a different address. > >   > > This PCI address mismatch breaks migration of existing guests > > to new libvirt versions. > >   > > Luckily, when QEMU has switched its default '-usb' controller > > from pci-ohci to nec-usb-xhci (QEMU commit 57040d451315), it > > has done so without affecting older machine types, which means > > we can keep using '-usb' without risking guest ABI breakage. > >   > > Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1357468 > > --- > >   src/qemu/qemu_command.c                                       | 8 +++++--- > >   tests/qemuxml2argvdata/qemuxml2argv-ppc64-usb-controller.args | 2 +- > >   tests/qemuxml2argvdata/qemuxml2argv-pseries-usb-default.args  | 2 +- > >   3 files changed, 7 insertions(+), 5 deletions(-) >  > Ján pointed out that applying this would un-break migration > for ppc64 guests created before 8156493d8db9, but at the > same time break migration for those created since, eg. in > the last ~6 months. >  > I'm kinda out of ideas here, so if anyone has a brilliant > plan to make migration work for both old and new guests, > please do share :)   It seems that we could solve this by just changing the logic in qemuDomainAssignDevicePCISlots() so that when it is auto-assigning addresses, it always uses 00:00.0 for the USB controller when guest arch is ppc64.   Existing guests deployed from a libvirt version using -device won't be affected, because we'll have recorded a PCI address for that in the XML and continue to use that. ie the auto-allocation logic won't run for them.   Existing guests deployed from a libvirt version using -usb should then get the fixed PCI address of 00:00.0 when they upgrade to the fix libvirt (assuming they didn't get run with a broken libvirt in between).

Regardless though, I'm very much against any change that tales us back to using -usb. This is long since obsolete syntax we should aim to never use - bringing ppc inline with other arches using -device is very much desirable.