On Wed, Jul 09, 2025 at 08:20:35AM +0000, Shameerali Kolothum Thodi wrote:
> > On Tue, Jul 08, 2025 at 04:40:50PM +0100, Shameer Kolothum wrote:
> > > @@ -2909,6 +2909,19 @@ static void
> > pci_device_get_iommu_bus_devfn(PCIDevice *dev,
> > > }
> > > }
> > >
> > > + /*
> > > + * When multiple PCI Express Root Buses are defined using pxb-
> > pcie,
> > > + * the IOMMU configuration may be specific to each root bus.
> > However,
> > > + * pxb-pcie acts as a special root complex whose parent is
> > effectively
> > > + * the default root complex(pcie.0). Ensure that we retrieve the
> > > + * correct IOMMU ops(if any) in such cases.
> > > + */
> > > + if (pci_bus_is_express(iommu_bus) &&
> > pci_bus_is_root(iommu_bus)) {
> > > + if (!iommu_bus->iommu_per_bus && parent_bus-
> > >iommu_per_bus) {
> > > + break;
> >
> > Mind elaborating why the current bus must unset iommu_per_bus while
> > its parent sets iommu_per_bus?
> >
> > My understanding is that for a pxb-pcie we should set iommu_per_bus
> > but for its parent (the default root complex) we should unset its
> > iommu_per_bus?
>
> Well, for new arm-smmuv3 dev you need an associated pcie root
> complex. Either the default pcie.0 or a pxb-pcie one. And as I
> mentioned in my reply to the other thread(patch #2) and commit log here,
> the pxb-pcie is special extra root complex in Qemu which has pcie.0 has
> parent bus.
>
> The above pci_device_get_iommu_bus_devfn() at present, iterate over the
> parent_dev if it is set and returns the parent_bus IOMMU ops even if the
> associated pxb-pcie bus doesn't have any IOMMU. This creates problem
> for a case that is described here in the cover letter here,
> https://lore.kernel.org/qemu-devel/20250708154055.101012-1-shameerali.kolothum.th...@huawei.com/
>
> (Please see "Major changes from v4:" section)
>
> To address that issue, this patch introduces an new helper function to
> specify that
> the IOMMU ops are specific to the associated root complex(iommu_per_bus) and
> use that to return the correct IOMMU ops.
>
> Hope with that context it is clear now.
Hmm, I was not questioning the context, I get what the patch is
supposed to do.
I was asking the logic that is unclear to me why it breaks when:
!pxb-pcie->iommu_per_bus && pcie.0->iommu_per_bus
Or in which case pcie.0 would be set to iommu_per_bus=true?
Thanks
Nicolin
