RE: [PATCH rc] iommu: Skip PASID validation for devices without PASID capability

[Tian, Kevin]

> From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> Sent: Thursday, April 24, 2025 11:27 AM
> 
> On 4/24/25 10:06, Tushar Dave wrote:
> > Generally PASID support requires ACS settings that usually create
> > single device groups, but there are some niche cases where we can get
> > multi-device groups and still have working PASID support. The primary
> > issue is that PCI switches are not required to treat PASID tagged TLPs
> > specially so appropriate ACS settings are required to route all TLPs to
> > the host bridge if PASID is going to work properly.
> >
> > pci_enable_pasid() does check that each device that will use PASID has
> > the proper ACS settings to achieve this routing.
> >
> > However, no-PASID devices can be combined with PASID capable devices
> > within the same topology using non-uniform ACS settings. In this case
> > the no-PASID devices may not have strict route to host ACS flags and
> > end up being grouped with the PASID devices.

Is there a detailed example?

> >
> > This configuration fails to allow use of the PASID within the iommu
> > core code which wrongly checks if the no-PASID device supports PASID.
> >
> > Fix this by ignoring no-PASID devices during the PASID validation. They
> > will never issue a PASID TLP anyhow so they can be ignored.
> >
> > Fixes: c404f55c26fc ("iommu: Validate the PASID in
> iommu_attach_device_pasid()")
> > Cc:stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Tushar Dave<tdave@xxxxxxxxxx>
> > ---
> >   drivers/iommu/iommu.c | 8 +++++++-
> >   1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > index 4f91a740c15f..e01df4c3e709 100644
> > --- a/drivers/iommu/iommu.c
> > +++ b/drivers/iommu/iommu.c
> > @@ -3440,7 +3440,13 @@ int iommu_attach_device_pasid(struct
> iommu_domain *domain,
> >
> >   	mutex_lock(&group->mutex);
> >   	for_each_group_device(group, device) {
> > -		if (pasid >= device->dev->iommu->max_pasids) {
> > +		/*
> > +		 * Skip PASID validation for devices without PASID support
> > +		 * (max_pasids = 0). These devices cannot issue transactions
> > +		 * with PASID, so they don't affect group's PASID usage.
> > +		 */
> > +		if ((device->dev->iommu->max_pasids > 0) &&
> > +		    (pasid >= device->dev->iommu->max_pasids)) {
> 
> What the iommu driver should do when set_dev_pasid is called for a non-
> PASID device? The iommu driver has no sense of iommu group, hence it has
> no knowledge about this device sharing an iommu group with another PASID
> capable device.

could add a similar check in __iommu_set_group_pasid() and
__iommu_remove_group_pasid() to skip those devices.