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

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

Just ping, any concern about this?

Thanks
Tiejun

> -----Original Message-----
> From: qemu-devel-bounces+tiejun.chen=intel.com@nongnu.org
> [mailto:qemu-devel-bounces+tiejun.chen=intel.com@nongnu.org] On Behalf Of
> Chen, Tiejun
> Sent: Wednesday, May 21, 2014 3:08 PM
> To: Gerd Hoffmann; Anthony PERARD; Daniel P. Berrange
> Cc: peter.maydell@linaro.org; xen-devel@lists.xensource.com;
> mst@redhat.com; stefano.stabellini@eu.citrix.com; Kay, Allen M;
> Kelly.Zytaruk@amd.com; qemu-devel@nongnu.org; Zhang, Yang Z;
> anthony@codemonkey.ws; anthony.perard@citrix.com
> Subject: Re: [Qemu-devel] [v2][PATCH 4/8] xen, gfx passthrough: reserve
> 00:02.0 for INTEL IGD
> 
> > -----Original Message-----
> > From: Gerd Hoffmann [mailto:kraxel@redhat.com]
> > Sent: Monday, May 19, 2014 9:51 PM
> > To: Chen, Tiejun
> > Cc: anthony.perard@citrix.com; stefano.stabellini@eu.citrix.com;
> > mst@redhat.com; Kelly.Zytaruk@amd.com; peter.maydell@linaro.org;
> > xen-devel@lists.xensource.com; Kay, Allen M; qemu-devel@nongnu.org;
> > anthony@codemonkey.ws; Zhang, Yang Z
> > Subject: Re: [Qemu-devel] [v2][PATCH 4/8] xen, gfx passthrough:
> > reserve
> > 00:02.0 for INTEL IGD
> >
> >   Hi,
> >
> > > > 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.
> >
> > libvirt does this unconditionally, because it is a good idea anyway
> > for a number of reasons.  Example: create a machine with three pci
> > devices, hot-unplug the second, then live-migrate to another machine.
> > The only way to create the correct config on the target machine is to
> > explicitly assign slots, otherwise the third pci device ends up in the wrong
> slot.
> >
> > Don't know how the libxl (and xl tool) work.  Maybe it does the same
> anyway.
> > Maybe it can handle the address assignment transparently for the user.
> >
> > > > 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.
> >
> > Going for (1) certainly is easier as (2) indeed will need changes in
> > the libxl, and might be tricky to get work with both old+new versions.
> >
> > > And especially, how to guarantee no one occupy 00:02.0 in the future
> > > with auto assign?
> >
> > Creating devices automatically turned out to have a number of problems.
> > So it is very unlikely we'll ever do this again.
> >
> 
> According to our discussions, I realize we may have some plans or policies
> dedicated to how to assign devfn, but to support GFX passthrough for XEN, I
> think currently it may be a better solution to adopt #1 simply like this:
> 
> diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c index eaf3e61..500b3c2
> 100644
> --- a/hw/i386/pc_piix.c
> +++ b/hw/i386/pc_piix.c
> @@ -386,7 +386,7 @@ static void pc_xen_hvm_init(QEMUMachineInitArgs
> *args)
> 
>      bus = pci_find_primary_bus();
>      if (bus != NULL) {
> -        pci_create_simple(bus, -1, "xen-platform");
> +        pci_create_simple(bus, PCI_DEVFN(3,0), "xen-platform");
>      }
>  }
>  #endif
> 
> Then we can go out to plan how to assign devfn in common, is this fine?
> 
> Thanks
> Tiejun