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

From: Jason Gunthorpe
Date: Tue May 04 2021 - 12:31:53 EST

On Tue, May 04, 2021 at 09:22:55AM -0700, Jacob Pan wrote:
> Hi Jason,
> On Wed, 28 Apr 2021 17:46:06 -0300, Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> > > > I think the name IOASID is fine for the uAPI, the kernel version can
> > > > be called ioasid_id or something.
> > >
> > > ioasid is already an id and then ioasid_id just adds confusion. Another
> > > point is that ioasid is currently used to represent both PCI PASID and
> > > ARM substream ID in the kernel. It implies that if we want to separate
> > > ioasid and pasid in the uAPI the 'pasid' also needs to be replaced with
> > > another general term usable for substream ID. Are we making the
> > > terms too confusing here?
> >
> > This is why I also am not so sure about exposing the PASID in the API
> > because it is ultimately a HW specific item.
> >
> > As I said to David, one avenue is to have some generic uAPI that is
> > very general and keep all this deeply detailed stuff, that really only
> > matters for qemu, as part of a more HW specific vIOMMU driver
> > interface.
> I think it is not just for QEMU. I am assuming you meant PASID is
> needed for guest driver to program assigned but not mediated devices.

Anything that directly operates the device and tries to instantiate
PASIDs for vfio-pci devices will need to understand the PASID.

> User space drivers may also need to get the real HW PASID to program it on
> to the HW. So this uAPI need to provide some lookup functionality. Perhaps
> the kernel generic version can be called ioasid_hw_id?
> So we have the following per my understanding:
> - IOASID: a userspace logical number which identifies a page table, this can
> be a first level (GVA-GPA), or a second level (GPA->HPA) page table.
> - PASID: strictly defined in PCIe term
> - Substream ID: strictly defined in ARM SMMUv3 spec.
> - IOASID_HW_ID: a generic ID backed by PASID, Substream ID, or any other
> HW IDs used to tag DMA
> Is that right?

It is reasonable. If a IOASID_HW_ID IOCTL can back with a enum that
qualified its exact nature it might be perfectly fine.