Re: [Qemu-devel] [Xen-devel] [PATCH v2] libxl: change default QEMU machine to pc-i440fx-1.6

[Stefano Stabellini]

On Fri, 11 Jul 2014, Ian Campbell wrote:
> On Fri, 2014-07-11 at 12:14 +0100, Stefano Stabellini wrote:
> > On Thu, 10 Jul 2014, Ian Jackson wrote:
> > > Stefano Stabellini writes ("Re: [PATCH v2] libxl: change default QEMU 
> > > machine to pc-i440fx-1.6"):
> > > > ping?
> > > > 
> > > > On Thu, 12 Jun 2014, Stefano Stabellini wrote:
> > > ...
> > > > > This patch does not change the emulated environment in the guest, 
> > > > > unless
> > > > > soundhw='hda' is specified, in that case the xen-platform device is
> > > > > moved to slot 3 (used to be always slot 2). This change might cause
> > > > > problems to guests with soundhw='hda', migrating from 4.4 to 4.5.
> > > 
> > > Is there a way for someone who experiences this problem to make it go
> > > back to the way it was ?
> > 
> > A guest reboot would solve the problem.
> 
> That rather defeats the purpose of being able to migrate from N to N+1.
> 
> What is the issue with hda which requires us to move slots around if it
> is enabled where we didn't need to do so before?

If we use -M pc or -M pc-i440fx-1.6, the xen-platform-pci device is
created slightly later in the QEMU boot sequence. The Intel HDA device
is able to "steal" its slot on the PCI bus.

On the other hand it is worth pointing out that currently libxl switches
from -M xenfv to -M pc silently depending on whether the
xen_platform_pci option is enabled or disabled. If it is disabled we get
pc, that is a moving target and not guaranteed to be compatible with
anything.