Re: [PATCH v6] xen/pt: reserve PCI slot 2 for Intel igd-passthru

[Chuck Zmudzinski <brchuckz@xxxxxxx>]

On 1/6/2023 10:02 AM, Chuck Zmudzinski wrote:
> On 1/6/23 9:31 AM, Chuck Zmudzinski wrote:
> > On 1/6/23 9:10 AM, Chuck Zmudzinski wrote:
> >> On 1/6/23 9:03 AM, Anthony PERARD wrote:
> >>> On Sun, Jan 01, 2023 at 06:52:03PM -0500, Chuck Zmudzinski wrote:
> >>>> ...
> >>>> 
> >>>> Signed-off-by: Chuck Zmudzinski <brchuckz@xxxxxxx>
> >>> 
> >>> 
> >>> This patch looks good enough. It only changes the "xenfv" machine so it
> >>> doesn't prevent a proper fix to be done in the toolstack libxl.
> >>> 
> >>> The change in xen_pci_passthrough_class_init() to try to run some code
> >>> before pci_qdev_realize() could potentially break in the future due to
> >>> been uncommon but hopefully that will be ok.
> >>> 
> >>> So if no work to fix libxl appear soon, I'm ok with this patch:

I have a patch that fixes it in libxl. It still needs a few tweaks before it is
ready for submission, but I plan to do that soon, perhaps later today or
tomorrow at the latest.

> > 
> > Well, I can tell you and others who use qemu are more comfortable
> > fixing this in libxl, so hold off for a week or so. I should have
> > a patch to fix this in libxl written and tested by then. If for
> > some reason that does not work out, then we can fix it in qemu.
>
> One last thought: the only donwnside to fixing this in libxl is that
> other toolstacks that configure qemu to use the xenfv machine will not
> benefit from the fix in qemu that would simplify configuring the
> guest correctly for the igd. Other toolstacks would still need to
> override the default behavior of adding the xen platform device at
> slot 2. I think no matter what, we should at least patch qemu to have
> the xen-platform device use slot 3 instead of being automatically assigned
> to slot 2 when igd-passthru=on. The rest of the fix could then be
> implemented in libxl so that other pci devices such as emulated network
> devices, other passed through pci devices, etc., do not take slot 2 when
> gfx_passthru in xl.cfg is set.

I decided to write the patch to libxl to fix this presuming no
changes to qemu. I think dealing with the "qemu behaves
differently starting from version 8 problem" is more trouble
that it's worth, so I am OK with implementing the fix completely
in libxl, which means libxl will now use the "pc" machine type
when igd-passthru=on and xen_platform_pci is true, but my patch
to libxl will still use the "xenfv" machine when xen_platform_pci
is true and igd-passthru is disabled.

Cheers,

Chuck