Re: [Qemu-devel] [PATCH] xen: make xen-platform a default device

[Paul Durrant]

> -----Original Message-----
> From: Stefano Stabellini [mailto:address@hidden
> Sent: 23 May 2014 11:11
> To: Paul Durrant
> Cc: Stefano Stabellini; Gerd Hoffmann; Paolo Bonzini; qemu-
> address@hidden; Anthony Liguori; address@hidden
> Subject: RE: [PATCH] xen: make xen-platform a default device
> 
> On Fri, 23 May 2014, Paul Durrant wrote:
> > > -----Original Message-----
> > > From: Stefano Stabellini [mailto:address@hidden
> > > Sent: 22 May 2014 14:57
> > > To: Gerd Hoffmann
> > > Cc: Paolo Bonzini; Stefano Stabellini; Paul Durrant; qemu-
> address@hidden;
> > > Anthony Liguori; address@hidden
> > > Subject: Re: [PATCH] xen: make xen-platform a default device
> > >
> > > On Thu, 22 May 2014, Gerd Hoffmann wrote:
> > > > > > Given that libxl always passes -nodefaults to QEMU, this patch is
> going
> > > > > > to effectively disable xen_platform_pci for all Xen users. It is 
> > > > > > not a
> > > > > > good idea. With the patch applied a Xen user would have no way to
> > > enable
> > > > > > xen_platform_pci except for passing some magic command line
> runes
> > > via
> > > > > > device_model_args_hvm.
> > > > >
> > > > > In fact this code only runs for "-M xenfv".  If you use "-M pc", the
> > > > > xen-platform device has to be added manually.  Perhaps it would be
> > > > > worthwhile to do the opposite, i.e. add the xen-platform device to "-
> M
> > > > > pc" if not using -nodefaults.
> > > >
> > > > /me looks at the code.  Yes, all the differences between xenfv and pc
> > > > machine types are guarded by if (xen_enabled()) these days, except
> for
> > > > adding the platform device.
> > > >
> > > > So using the pc machine type should just work on xen, and give you a
> > > > machine without the platform device.  So it can be added via -device, at
> > > > any slot, if needed.  No need to patch qemu at all.  Adding or not
> > > > adding xen-platform can easily handled by libxl then, depending on the
> > > > xen_platform_pci switch in the config file.
> > >
> > > I agree. Changing libxl to always use -M pc and using -device to add
> > > xen-platform when needed sounds like the best option.
> >
> > Really? You opposed this before:
> >
> > http://lists.xen.org/archives/html/xen-devel/2013-06/msg01946.html
> >
> > stating that you wanted a mechanism to query the running version of
> QEMU before choosing the machine type.
> 
> Many QEMU releases have been made since then. I feel more comfortable
> with using -M pc, especially given the alternatives that have been
> proposed. Of course having a mechanism to query the running version of
> QEMU would still be the safest solution.

But I thought the problem was that, should someone change what 'pc' means, then 
libxl would invoke the 'wrong' machine type on a domain restore. I know that 
xenfv is essentially 'pc' at the moment but would could fix on a particular 
version if compatibility becomes an issue. So, rather than choosing 'pc' now, 
should we not choose 'pc-i440fx-1.6' which is the current default?

  Paul