Re: [PATCH v2 1/2] PCI: Allow per function PCI slots

Niklas Schnelle
2025-10-29T08:37:20Z

On Tue, 2025-10-28 at 09:11 +0800, Bibo Mao wrote:
>
> On 2025/10/27 下午8:29, Niklas Schnelle wrote:
> > On Wed, 2025-10-22 at 14:24 -0700, Farhan Ali wrote:
> > > On s390 systems, which use a machine level hypervisor, PCI devices are
> > > always accessed through a form of PCI pass-through which fundamentally
> > > operates on a per PCI function granularity. This is also reflected in the
> > > s390 PCI hotplug driver which creates hotplug slots for individual PCI
> > > functions. Its reset_slot() function, which is a wrapper for
> > > zpci_hot_reset_device(), thus also resets individual functions.
> > >
> > > Currently, the kernel's PCI_SLOT() macro assigns the same pci_slot object
> > > to multifunction devices. This approach worked fine on s390 systems that
> > > only exposed virtual functions as individual PCI domains to the operating
> > > system. Since commit 44510d6fa0c0 ("s390/pci: Handling multifunctions")
> > > s390 supports exposing the topology of multifunction PCI devices by
> > > grouping them in a shared PCI domain. When attempting to reset a function
> > > through the hotplug driver, the shared slot assignment causes the wrong
> > > function to be reset instead of the intended one. It also leaks memory as
> > > we do create a pci_slot object for the function, but don't correctly free
> > > it in pci_slot_release().
> > >
> > > Add a flag for struct pci_slot to allow per function PCI slots for
> > > functions managed through a hypervisor, which exposes individual PCI
> > > functions while retaining the topology.
> >
> > I wonder if LoongArch which now also does per PCI function pass-through
> > might need this too. Adding their KVM maintainers.
>
> Hi Niklas,
>
> Thanks for your reminder. Yes, LoongArch do per PCI function
> pass-throught. In theory, function pci_slot_enabled_per_func() should
> return true on LoongArch also. Only that now IOMMU driver is not merged,
> there is no way to test it, however we will write down this as a note
> inside about this issue and verify it once IOMMU driver is merged.
>
>
> Regards
> Bibo Mao
>

Hi Bibo,

Is your ability to test if it works for you also hindered for my other
patch touching pci_scan_slot()? It sounded like Huacai was looking into
testing that.

Thanks,
Niklas