Re: Potential issue with hypervisor_isolated_pci_functions() handling

Huacai Chen
2025-10-22T14:49:25Z

Hi, Niklas,

On Wed, Oct 22, 2025 at 7:40 PM Niklas Schnelle <schnelle@linux.ibm.com> wrote:
>
> Hi Huacai, Hi Jan, Hi Bjorn,
>
> I noticed that with commit a02fd05661d7 ("PCI: Extend isolated function
> probing to LoongArch") LoongArch now also makes use of the isolated
> function probing. First, nice to see this sees more users and that the
> interface is useful to you.
>
> Seeing this, I was reminded of an issue I ran into when using this
> mechanism with SR-IOV capable devices. In that case VFs with devfn > 7
> and PCI_SLOT(devfn) != 0 wouldn't get probed.
> This is because if a device is found next_fn() will check whether dev-
> >multifunction is set. So if for example devfn == 8 is found (fn == 0)
> dev->multifunction won't be set in pci_scan_slot() and for VFs it's
> also not set via PCI_HEADER_TYPE_MFD. So we won't check for devfn == 9
> even though that could very well be there.
>
> Now for s390 this currently doesn't cause issues because for all
> multifunction devices we have, we either get a VF alone and then since
> commit 25f39d3dcb48 ("s390/pci: Ignore RID for isolated VFs") use devfn
> == 0, or we have the parent PF passed-through and then VFs always get
> hot plugged after SR-IOV enable, which then uses
> pci_scan_single_device(). And for non VFs we always have devfn == 0
> and/or devfn == 1 and multifunction via the header. So in a sense the
> above commit works around the issue, though that wasn't its primary
> intention.
>
> Did either of you also run into this issue or can reproduce it?
>
> Somewhat related, if ARI is enabled this would also break the isolated
> function case including on s390 where ARI is used by the platform
> firmware, but detected as off by Linux because there is no struct
> pci_dev associated with the PCI bus. If I patch Linux to correctly
> detect ARI, it no longer finds an isolated PF with devfn == 1.
LoongArch do have some problems after commit a02fd05661d7 ("PCI:
Extend isolated function probing to LoongArch"). Please see:
https://lore.kernel.org/linux-pci/20251014074100.2149737-1-chenhuacai@loongson.cn/

Now we don't know what happens exactly, so I haven't answered Bjorn's
question...

Huacai
>
> Thanks,
> Niklas Schnelle