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

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Jul 01, 2025 at 01:29:05PM -0600, Alex Williamson wrote:
> On Mon, 30 Jun 2025 19:28:33 -0300
> Jason Gunthorpe <jgg@nvidia.com> wrote:
> > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > index d265de874b14b6..f4584ffacbc03d 100644
> > --- a/drivers/iommu/iommu.c
> > +++ b/drivers/iommu/iommu.c
> > @@ -65,8 +65,16 @@ struct iommu_group {
> >  	struct list_head entry;
> >  	unsigned int owner_cnt;
> >  	void *owner;
> > +
> > +	/* Used by the device_group() callbacks */
> > +	u32 bus_data;
> >  };
> >  
> > +/*
> > + * Everything downstream of this group should share it.
> > + */
> > +#define BUS_DATA_PCI_UNISOLATED BIT(0)
> 
> NON_ISOLATED for consistency w/ enum from the previous patch?

Yes

> > -	/* No shared group found, allocate new */
> > -	return iommu_group_alloc();
> > +	switch (pci_bus_isolated(pdev->bus)) {
> > +	case PCIE_ISOLATED:
> > +		/* Check multi-function groups and same-bus devfn aliases */
> > +		group = pci_get_alias_group(pdev);
> > +		if (group)
> > +			return group;
> > +
> > +		/* No shared group found, allocate new */
> > +		return iommu_group_alloc();
> 
> I'm not following how we'd handle a multi-function root port w/o
> consistent ACS isolation here.  How/where does the resulting group get
> the UNISOLATED flag set?

Still wobbly on the root port/root bus.. So the answer is probably
that it doesn't.

What does a multi-function root port with different ACS flags even
mean and how should we treat it? I had in mind that the first root
port is the TA and immediately goes the IOMMU.

If you can explain a bit more about how you see the root ports working
I can try to make an implementation.

AFAICT the spec sort of says 'implementation defined' for ACS on root
ports??

Thanks,
Jason