RE: [PATCH 1/1] iommu/vt-d: Enable PASID during iommu device probe

[Tian, Kevin]

> From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> Sent: Monday, September 12, 2022 10:48 AM
> 
> Previously PASID supports on both IOMMU and PCI devices are enabled in
> the
> iommu_dev_enable_feature(dev, IOMMU_DEV_FEAT_SVA) path. It's
> functionally
> correct as the SVA is the only feature that requires PASID setup. However,
> looking ahead, we will add more features that need to enable pasid (for
> example, kernel DMA with PASID, SIOV, VM guest SVA, etc.). It makes more
> sense to enable PASID during iommu probing device.
> 
> This enables PASID during iommu probing device and deprecates the
> intel_iommu_enable_pasid() helper. This is safe because the IOMMU
> hardware
> will block any PCI TLP with a PASID prefix if there is no IOMMU domain
> attached to the PASID of the device.
> 

IMHO it's better to enable something only when it's actually required,
e.g. does it make more sense to have a IOMMU_DEV_FEAT_PASID
instead?

What this patch does has one problem. It's an intel-iommu driver
internal policy. How can a device driver reliably tell that the pasid
capability has been enabled by the iommu driver?

Thanks
Kevin