RE: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

From: Jason Gunthorpe <redacted>
Sent: Wednesday, March 31, 2021 8:41 PM

On Wed, Mar 31, 2021 at 07:38:36AM +0000, Liu, Yi L wrote:

quoted

The reason is /dev/ioasid FD is per-VM since the ioasid allocated to
the VM should be able to be shared by all assigned device for the VM.
But the SVA operations (bind/unbind page table, cache_invalidate) should
be per-device.

It is not *per-device* it is *per-ioasid*

And as /dev/ioasid is an interface for controlling multiple ioasid's
there is no issue to also multiplex the page table manipulation for
multiple ioasids as well.

What you should do next is sketch out in some RFC the exactl ioctls
each FD would have and show how the parts I outlined would work and
point out any remaining gaps.

The device FD is something like the vfio_device FD from VFIO, it has
*nothing* to do with PASID beyond having a single ioctl to authorize
the device to use the PASID. All control of the PASID is in
/dev/ioasid.

good to see this reply. Your idea is much clearer to me now. If I'm getting
you correctly. I think the skeleton is something like below:

1) userspace opens a /dev/ioasid, meanwhile there will be an ioasid
   allocated and a per-ioasid context which can be used to do bind page
   table and cache invalidate, an ioasid FD returned to userspace.
2) userspace passes the ioasid FD to VFIO, let it associated with a device
   FD (like vfio_device FD).
3) userspace binds page table on the ioasid FD with the page table info.
4) userspace unbinds the page table on the ioasid FD
5) userspace de-associates the ioasid FD and device FD

Does above suit your outline?

If yes, I still have below concern and wish to see your opinion.
- the ioasid FD and device association will happen at runtime instead of
  just happen in the setup phase.
- how about AMD and ARM's vSVA support? Their PASID allocation and page table
  happens within guest. They only need to bind the guest PASID table to host.
  Above model seems unable to fit them. (Jean, Eric, Jacob please feel free
  to correct me)
- this per-ioasid SVA operations is not aligned with the native SVA usage
  model. Native SVA bind is per-device.

Regards,
Yi Liu