Re: [PATCH v11 6/7] xen/arm: enable dom0 to use PCI devices with pci-passthrough=no

[Mykyta Poturai <Mykyta_Poturai@xxxxxxxx>]

On 04.06.25 08:52, Jan Beulich wrote:
> On 03.06.2025 15:31, Mykyta Poturai wrote:
>> On 02.06.25 11:11, Jan Beulich wrote:
>>> On 28.05.2025 11:12, Mykyta Poturai wrote:
>>>> From: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
>>>>
>>>> Enable the use of IOMMU + PCI in dom0 without having to specify
>>>> "pci-passthrough=yes". Due to possible platform specific dependencies
>>>> of the PCI host, we rely on dom0 to initialize it and perform
>>>> a PHYSDEVOP_pci_device_add/remove call to add each device to SMMU.
>>>> PHYSDEVOP_pci_device_reset is left untouched as it does not have the
>>>> pci_passthrough_enabled check.
>>>
>>> Just to re-raise the question here: Is this actually correct?
>>
>> I'm afraid I don't quite understand your concerns here.
>>
>> The purpose of this patch is to relax the pci_passthrough_enabled checks
>> and make PCI physdev ops work with passthrough disabled.
>> The reset op worked independently of PCI passthrough being on or off and
>> will continue to do so after this patch.
>> If your concerns are about the correctness of allowing reset to always
>> work, you specifically requested this behavior in the patches
>> implementing it here[1].
> 
> Right, yet even there I had already asked for possible differing opinions.
> Plus the case I had mentioned was specifically Dom0, which fits here.
> 
> Jan

So I've done some testing to see the actual behavior with different 
combinations of pci-passthrough and iommu switches. With passthrough=off 
and iommu=on the reset works fine. But with both of them off, it fails 
because PHYSDEVOP_pci_device_add is not adding anything and therefore 
pci_get_pdev can't find the pdev.

I am not sure which behavior would be the correct one here for 
passthrought=off and iommu=off.

1. Leave it as is, reset returns -ENODEV and pciback probe fails
2. Add the same check as in add/remove, reset will return -EOPNOTSUPP 
and pciback probe will also fail
3. Add the same check as in add/remove but return 0 so pciback can probe 
the device.

Maybe you have some thoughts on this. I can't come up with an actual 
good reason for using pciback without pci-passthrough enabled, outside 
of maybe "not breaking some abstract scripts". And EOPNOTSUPP seems more 
descriptive than ENODEV so I strive towards option 2 if everyone okay 
with that.


-- 
Mykyta