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

[Tian, Kevin]

> From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> Sent: Tuesday, September 13, 2022 2:01 PM
> 
> Hi Kevin,
> 
> On 2022/9/13 11:13, Tian, Kevin wrote:
> >> 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?
> 
> PASID is a capability (not a feature) of a device. Hence from my point
> of view, the IOMMU driver could enable it by default as long as the
> IOMMU can handle transactions with PASID. Currently other PCIe
> capabilities like ATS and PRI are also handled in this way.
> 

OK, then it makes some sense. and later the PASID capability can
be reported so a driver can query to enable certain usage (e.g. SIOV)
based on it.