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

[Chuck Zmudzinski <brchuckz@xxxxxxx>]

On 1/6/23 5:52 AM, Anthony PERARD wrote:
> On Tue, Jan 03, 2023 at 05:58:01PM -0500, Chuck Zmudzinski wrote:
>> Hello Anthony and Paul,
> 
> Hi Chuck,
> 
>> I am requesting your feedback to Alex Williamson's suggestion that this
>> problem with assigning the correct slot address to the igd on xen should
>> be fixed in libxl instead of in qemu.
>> 
>> It seems to me that the xen folks and the kvm folks have two different
>> philosophies regarding how a tool stack should be designed. kvm/libvirt
>> provides much greater flexibility in configuring the guest which puts
>> the burden on the administrator to set all the options correctly for
>> a given feature set, while xen/xenlight does not provide so much
>> flexibility and tries to automatically configure the guest based on
>> a high-level feature option such as the igd-passthrough=on option that
>> is available for xen guests using qemu but not for kvm guests using
>> qemu.
>> 
>> What do you think? Should libxl be patched instead of fixing the problem
>> with this patch to qemu, which is contrary to Alex's suggestion?
> 
> I do think that libxl should be able to deal with having to put a
> graphic card on slot 2. QEMU already provides every API necessary for a
> toolstack to be able to start a Xen guest with all the PCI card in the
> right slot. But it would just be a bit more complicated to implement in
> libxl.
> 
> At the moment, libxl makes use of the QEMU machine 'xenfv', libxl should
> instead start to use the 'pc' machine and add the "xen-platform" pci
> device. (libxl already uses 'pc' when the "xen-platform" pci card isn't
> needed.) Also probably add the other pci devices to specific slot to be
> able to add the passthrough graphic card at the right slot.
> 
> Next is to deal with migration when using the 'pc' machine, as it's just
> an alias to a specific version of the machine. We need to use the same
> machine on the receiving end, that is start with e.g. "pc-i440fx-7.1" if
> 'pc' was an alias for it at guest creation.
> 
> 
> I wonder if we can already avoid to patch the 'xenfv' machine with some
> xl config:
>     # avoid 'xenfv' machine and use 'pc' instead
>     xen_platform_pci=0
>     # add xen-platform pci device back
>     device_model_args_hvm = [
>         "-device", "xen-platform,addr=3",
>     ]
> But there's probably another device which is going to be auto-assigned
> to slot 2.
> 
> 
> If you feel like dealing with the technical dept in libxl, that is to
> stop using 'xenfv' and use 'pc' instead, then go for it, I can help with
> that. Otherwise, if the patch to QEMU only changes the behavior of the
> 'xenfv' machine then I think I would be ok with it.
> 
> I'll do a review of that QEMU patch in another email.
> 
> Cheers,
> 

Hello Anthony,

Thanks for responding!

The first part of my v6 of the patch only affects the xenfv
machine. Guests created with the pc machine type will not call
the new function that reserves slot 2 for the igd because that
function is only called when the machine type is xenfv (or xenfv-4.2).
But the new functions I added to configure the TYPE_XEN_PT_DEVICE
when igd-passthru=on will be called to check if the device is an
Intel igd and clear the slot if it is, but this will not have any
effect on the behavior in this case because the slot was never
reserved. Still, this would add some unnecessary processing in the
case of machines other than xenfv, which is undesirable.

So I can add a check for the machine type to a v7 of the patch
that will skip the new functions that clear the reserved slot if
slot 2 is not reserved and therefore does not need to be cleared.

Would that be OK?

Kind regards,

Chuck