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

[Baolu Lu]

On 2022/9/16 11:35, Ethan Zhao wrote:
> Baolu,
>
> 在 2022/9/16 11:05, Baolu Lu 写道:
> > On 2022/9/16 10:40, Ethan Zhao wrote:
> > > > I may not get you exactly. 😄 Some IOMMU features reply on PASID
> > > > capabilities on both IOMMU and device. The IOMMU drivers enumerate the
> > > > capabilities and enable them if they are supported.
> > > I might not express it straightforward,  I mean with this patch iommu 
> > > deals with
> > >
> > > the complexity of enabling PASID (globally?)  or not at probing stage 
> > > , instead
> > >
> > > of other device driver side decision to request IOMMU PASID enabling 
> > > during
> > >
> > > their setup state.  if so you move the decision to iommu probe stage. 
> > > hmmm...
> >
> > I am sorry that the commit message was a bit confusing. Actually we
> > always enable PASID at iommu probe path w/ or w/o this patch.
> Really ?  the commit message is quit clear to me ~~@
> > > Pros,  iommu driver controls everything about PASID enabling.
> > >
> > > Cons,  iommu driver handles all possible complexity about capability 
> > > matching
> >
> > Do device drivers need to configure PCI PASID without IOMMU? I searched
> > the tree and found nothing.
>
> Device knows if it has PCI PASID cap and its driver also could determine 
> to request
>
> iommu to enable PASID or not by invoking
>
> intel_iommu_enable_sva()->*intel_iommu_enable_pasid()*

PASID is a PCIe capability. Though SVA is built on it,it's not only
for SVA. Thus, the purpose of intel_iommu_enable_sva() is not for
enabling PASID.

> *that is the old style without this patch.

No. Without this patch, PASID is also enabled in probe path. Calling
intel_iommu_enable_pasid() in enabling SVA path is actually duplicate.

The commit message for this patch is not correct. It's my bad. :-)

> *
>
> *Iommu driver of course  also knows if devices in its group related the 
> iommu
> *
>
> *have PASID cap support or not by enumerating them from the ACPI DMAR.
> *
>
> This is my understanding, correct me if wrong.
>
> While configuring device PCI PASID cap is another thing, from kernel or 
> userland,
>
> compatible with the iommu cap, works, or not work.  Could you prevent anyone
>
> from doing that ?

I do not object to adding a common interface to enable/disable PASID if
any device driver needs to manage PASID by itself. Before that, there
is no need to add complexity in IOMMU subsystem for a non-existent
requirement.

Best regards,
baolu