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

[Stefano Stabellini]

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.