Re: [RFC v16 1/9] iommu: Introduce attach/detach_pasid_table API

[Jason Gunthorpe]

On Thu, Dec 09, 2021 at 03:59:57AM +0000, Tian, Kevin wrote:
> > From: Tian, Kevin
> > Sent: Thursday, December 9, 2021 10:58 AM
> > 
> > For ARM it's SMMU's PASID table format. There is no step-2 since PASID
> > is already within the address space covered by the user PASID table.
> > 
> 
> One correction here. 'no step-2' is definitely wrong here as it means
> more than user page table in your plan (e.g. dpdk).
> 
> To simplify it what I meant is:
> 
> iommufd reports how many 'user page tables' are supported given a device.
> 
> ARM always reports only one can be supported, and it must be created in 
> PASID table format. tagged by RID.
> 
> Intel reports one in step1 (tagged by RID), and N in step2 (tagged by
> RID+PASID). A special flag in attach call allows the user to specify the
> additional PASID routing info for a 'user page table'.

I don't think 'number of user page tables' makes sense

It really is 'attach to the whole device' vs 'attach to the RID' as a
semantic that should exist 

If we imagine a userspace using kernel page tables it certainly makes
sense to assign page table A to the RID and page table B to a PASID
even in simple cases like vfio-pci.

The only case where userspace would want to capture the entire RID and
all PASIDs is something like this ARM situation - but userspace just
created a device specific object and already knows exactly what kind
of behavior it has.

So, something like vfio pci would implement three uAPI operations:
 - Attach page table to RID
 - Attach page table to PASID
 - Attach page table to RID and all PASIDs
   And here 'page table' is everything below the STE in SMMUv3

While mdev can only support:
 - Access emulated page table
 - Attach page table to PASID

It is what I've said a couple of times, the API the driver calls
toward iommufd to attach a page table must be unambiguous as to the
intention, which also means userspace must be unambiguous too.

Jason