Re: [Xen-devel] [Qemu-devel] [v2][PATCH 4/8] xen, gfx passthrough: reserve 00:02.0 for INTEL IGD

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Mon, May 19, 2014 at 12:04:14PM +0000, Chen, Tiejun wrote:
> > -----Original Message-----
> > From: Gerd Hoffmann [mailto:kraxel@xxxxxxxxxx]
> > Sent: Monday, May 19, 2014 7:23 PM
> > To: Chen, Tiejun
> > Cc: anthony.perard@xxxxxxxxxx; stefano.stabellini@xxxxxxxxxxxxx;
> > mst@xxxxxxxxxx; Kelly.Zytaruk@xxxxxxx; peter.maydell@xxxxxxxxxx;
> > xen-devel@xxxxxxxxxxxxxxxxxxx; Kay, Allen M; qemu-devel@xxxxxxxxxx;
> > jean.guyader@xxxxxxxxxxxxx; anthony@xxxxxxxxxxxxx; Zhang, Yang Z
> > Subject: Re: [Qemu-devel] [v2][PATCH 4/8] xen, gfx passthrough: reserve
> > 00:02.0 for INTEL IGD
> > 
> >   Hi,
> > 
> > > I think '-vga none' just guarantees the qemu vga cards doesn't occupy
> > > 00:02.0, but this doesn't mean others use this specific slot since in
> > > qemu internal, we always pass -1 to assign a slot automatically to
> > > register a PCI device. So in some cases, we can't get this slot as we
> > > expect since that is already assigned previously before we need this.
> > 
> > Yes, -vga, -net nic, -drive if=scsi (maybe more) can internally create pci 
> > devices
> > with auto slot assignment, which will occupy slot 2 indeed.
> > Use -device instead to create the devices.
> > 
> 
> Are you saying we have to create the devices explicitly when we want to work 
> IGD vga with passthrough? But how to make sure all user know this workable 
> way? Maybe you suggest we should document somewhere.
> 
> > > PCI: slot 2 function 0 not available for xen-pci-passthrough, in use
> > > by xen-platform
> > 
> > Ah, the xen platform device.  /me looks.  Ah, pc_xen_hvm_init creates this
> > automatically.  Two options here IMHO:
> > 
> >   (1) Just move it somewhere else explicitly.  For example slot 3, or
> >       make it a southbridge function (say 00:01.7).
> >   (2) Don't create it automatically, instead expect management add it
> >       if needed, using -device xen-plaform,addr=...
> > 
> > I personally would suggest to go for #2.  As far I know the platform device 
> > is
> > only needed if you want attach xenbus devices to the guest (correct?), so
> > creating virtual machines without the xen platform device is a valid use 
> > case
> > and you should allow it.
> > 
> 
> Looks you recommend we should change current xen platform design, I'm not 
> sure if something in libxl also need to be modified. Especially, this may not 
> be compatible with those old xen version. And especially, how to guarantee no 
> one occupy 00:02.0 in the future with auto assign?

Hi,

Libxl knows how to create a guest without xen-platform, but I think this
can work only from QEMU 1.6 (which Xen 4.4 use).

So, using the options below with libxl would put the xen-platform device
away from the 00:02 slot.
xen_platform_pci=0
device_model_args_hvm = [ '-device', 'xen-platform,addr=0x3' ]


-- 
Anthony PERARD

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