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

[Daniel P. Berrange]

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.

Regards,
Daniel
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