Re: [PATCH v3 03/11] iommu: Compute iommu_groups properly for PCIe switches

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Sep 09, 2025 at 03:27:02PM -0500, Bjorn Helgaas wrote:
> > Instead the current algorithm always creates unique single device groups
> > for this topology. It happens because the pci_device_group(DSP)
> > immediately moves to the USP and computes pci_acs_path_enabled(USP) ==
> > true and decides the DSP can get a unique group. The pci_device_group(A)
> > immediately moves to the DSP, sees pci_acs_path_enabled(DSP) == false and
> > then takes the DSPs group.
> 
> s/takes the DSPs group/takes the DSP's group/ (I guess?)

yeah

> > While ACS on root ports is underspecified in the spec, it should still
> > function as an egress control and limit access to either the MMIO of the
> > root port itself, or perhaps some other devices upstream of the root
> > complex - 00:17.0 perhaps in this example.
> 
> Does ACS have some kind of MMIO-specific restriction? 

I guess no, the text could be more generic here.

> > As grouping is a security property for VFIO creating incorrectly narrowed
> > groups is a security problem for the system.
> 
> I.e., we treated devices as being isolated from P2PDMA when they
> actually were not isolated, right?  More isolation => smaller
> (narrower) IOMMU groups?

Yes

> > Revise the design to solve these problems.
> > 
> > Explicitly require ordering, or return EPROBE_DEFER if things are out of
> > order. This avoids silent errors that created smaller groups and solves
> > problem #1.
> 
> If it's easy to state, would be nice to say what ordering is required.
> The issue mentioned above was "discovering a downstream device before
> its upstream", so I guess you want to discover upstream devices before
> downstream?  

yes

> Obviously PCI enumeration already works that way, so
> IOMMU group discovery must be a little different.
 
iommu group discovery is driven off of iommu probing which can happen
in enough different ways that it needs to be checked.


I will fix the other notes

Thanks,
Jason