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

["Chen, Tiejun" <tiejun.chen@xxxxxxxxx>]

> -----Original Message-----
> From: Michael S. Tsirkin [mailto:mst@xxxxxxxxxx]
> Sent: Monday, May 19, 2014 6:13 PM
> To: Chen, Tiejun
> Cc: Gerd Hoffmann; anthony.perard@xxxxxxxxxx; 
> stefano.stabellini@xxxxxxxxxxxxx; 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
> 
> On Mon, May 19, 2014 at 09:25:19AM +0000, Chen, Tiejun wrote:
> > > -----Original Message-----
> > > From: Gerd Hoffmann [mailto:kraxel@xxxxxxxxxx]
> > > Sent: Monday, May 19, 2014 2:45 PM
> > > To: Chen, Tiejun
> > > Cc: anthony.perard@xxxxxxxxxx; stefano.stabellini@xxxxxxxxxxxxx; 
> > > mst@xxxxxxxxxx; Kelly.Zytaruk@xxxxxxx; peter.maydell@xxxxxxxxxx; 
> > > xen-devel@xxxxxxxxxxxxxxxxxxx; weidong.han@xxxxxxxxx; 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,
> > >
> > > > +    /*
> > > > +     * Some video bioses and gfx drivers will assume the bdf of 
> > > > + IGD is
> > > 00:02.0.
> > > > +     * So user need to set it to 00:02.0 in Xen configure file 
> > > > explicitly,
> > > > +     * otherwise IGD will fail to work.
> > > > +     */
> > > > +    pci_reserve_pci_devfn(b, PCI_DEVFN(2, 0));
> > >
> > > That is asking for trouble.  Slot 2 is used by the qemu vga cards 
> > > by default, and for quite a while (before memory api was merged) 
> > > it even was impossible to change it.  libvirt still places the vga 
> > > card at slot
> > > 2 for that reason -> boom.  I wouldn't be surprised if you find 
> > > that assumption in other management libs / apps too.
> > >
> > > Why do you need that patch in the first place?  It should be 
> > > possible to configure qemu to not occupy slot 2 if you need it 
> > > that way.  Just pass '-vga none' to qemu.  Which you probably want 
> > > anyway if you pass-through a vga to the guest.  And explicitly 
> > > configure a slot (via addr=
> >
> > 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.
> 
> So stop doing this.
> Address -1 is a short-cut intended for humans.

Are you saying we need remove all auto assigned places like this,

piix.c: i440fx_init()

    if (xen_enabled()) {
        piix3 = DO_UPCAST(PIIX3State, dev,
                pci_create_simple_multifunction(b, -1, true, "PIIX3-xen"));
        pci_bus_irqs(b, xen_piix3_set_irq, xen_pci_slot_get_pirq,
                piix3, XEN_PIIX_NUM_PIRQS);
    } else {
        piix3 = DO_UPCAST(PIIX3State, dev,
                pci_create_simple_multifunction(b, -1, true, "PIIX3"));

> In particular we have no way to guarantee that migration works unless 
> you specify addresses explicitly.
> 
> > > property) for all your pci devices.  Doing it only for the IGD 
> > > works too if you list the device before any other pci device on 
> > > the qemu command
> line.
> >
> > So in my test scenario, we can see this information:
> >
> > PCI: slot 2 function 0 not available for xen-pci-passthrough, in use 
> > by xen-platform Thanks Tiejun
> 
> I think the best fix is to give priority to devices that supply a specific 
> slot.
> 

Maybe I can extend that bitmap I implemented in patch #1 to do this.

But anyway you also need to concern if this is compatible with libxl in xen 
case. 

Thanks
Tiejun

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