Re: [Xen-devel] [Qemu-devel] [PATCH] Remove hardcoded xen-platform device initialization

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: qemu-devel-bounces+paul.durrant=citrix.com@xxxxxxxxxx
> [mailto:qemu-devel-bounces+paul.durrant=citrix.com@xxxxxxxxxx] On
> Behalf Of Paul Durrant
> Sent: 14 June 2013 10:01
> To: Paolo Bonzini; Ian Campbell
> Cc: xen-devel@xxxxxxxxxxxxx; qemu-devel@xxxxxxxxxx; Stefano Stabellini
> Subject: Re: [Qemu-devel] [Xen-devel] [PATCH] Remove hardcoded xen-
> platform device initialization
> 
> > -----Original Message-----
> > From: Paolo Bonzini [mailto:paolo.bonzini@xxxxxxxxx] On Behalf Of Paolo
> > Bonzini
> > Sent: 13 June 2013 19:03
> > To: Ian Campbell
> > Cc: Stefano Stabellini; Paul Durrant; qemu-devel@xxxxxxxxxx; xen-
> > devel@xxxxxxxxxxxxx
> > Subject: Re: [Xen-devel] [PATCH] Remove hardcoded xen-platform device
> > initialization
> >
> > Il 13/06/2013 13:44, Ian Campbell ha scritto:
> > > On Thu, 2013-06-13 at 18:33 +0100, Stefano Stabellini wrote:
> > >> On Thu, 13 Jun 2013, Paul Durrant wrote:
> > >>> The xen-platform device should be initialized by the Xen toolstack by
> > >>> passing the appropriate -device argument on the command line.
> > >>>
> > >>> Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > >>
> > >> This patch is problematic because we can't know for sure the version of
> > >> upstream QEMU that is going to be used with Xen.
> > >> If we apply this patch and QEMU 1.5 is going to be used with Xen 4.2,
> > >> guests won't be able to use PV drivers.
> > >
> > > Is the right answer a lever to disable, rather than enable, it?
> > >
> > > A workaround for the situation you envisage is to use the
> > > device_model_args config option, not ideal though.
> >
> > I think the right solution for this is to move towards using the normal
> > "-M pc" machine.  libxl can simply use "-M pc -machine accel=xen -device
> > xen-platform-pv"; older versions that use the xenfv machine will still work.
> >
> > And if you do this, you will also get the benefit of versioned machine
> > types.
> >
> 
> Thanks. I'll have a look at that.
> 

It's a little more complicated than I thought. There are two machine types, 
xenpv and xenfv (i.e. HVM), and they share the accel=xen option. Thus QEMU 
attaches to the VM in the machine code rather than the accelerator init code. 
Using -M pc is therefore not an option, unless we perhaps have separate accel 
options. Either way this doesn't address the backwards compatibility issue. I 
think QEMU is going to need to know which version of the Xen toolstack invoked 
it unless we specify a compatibility matrix.

  Paul
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