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

[Andrea Bolognani]

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 :)

-- 
Andrea Bolognani / Red Hat / Virtualization