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From: Gerd Hoffmann <kraxel@redhat.com>
Date: Thu, 22 May 2014 15:22:48 +0200
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Subject: Re: [Qemu-devel] [PATCH] xen: make xen-platform a default device
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Paul Durrant <paul.durrant@citrix.com>, mst@redhat.com, qemu-devel@nongnu.org, Anthony Liguori <aliguori@amazon.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>


> > 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.

cheers,
  Gerd