Re: Potential issue with hypervisor_isolated_pci_functions() handling

Huacai Chen
2025-10-24T10:05:33Z

Hi, Niklas,

On Thu, Oct 23, 2025 at 11:25 PM Niklas Schnelle <schnelle@linux.ibm.com> wrote:
>
> On Wed, 2025-10-22 at 22:49 +0800, Huacai Chen wrote:
> > 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
> >
>
> Hi Huacai,
>
> I have now send[0] a fix proposal together with correctly exposing ARI
> enablement in s390. I also reproduced both the issue I saw with SR-IOV
> and with ARI and confirmed that the first patch in the series fixes
> them. You're on Cc and I'd highly appreciate if you could test whether
> this fixes your issues as well.
>
> Thanks,
> Niklas
>
> [0]
> https://lore.kernel.org/linux-pci/20251023-ari_no_bus_dev-v4-0-8482e2ed10bd@linux.ibm.com/

There are two patches in this series. If LoongArch has a similar
problem as S390, only the first patch is enough, or should we need an
architectural change similar to the second patch?

Huacai