Re: [RFC v2] /dev/iommu uAPI proposal

Jason Gunthorpe
2021-07-22T16:30:44Z

On Wed, Jul 21, 2021 at 02:13:23AM +0000, Tian, Kevin wrote:
> > From: Shenming Lu
> > Sent: Friday, July 16, 2021 8:20 PM
> >
> > On 2021/7/16 9:20, Tian, Kevin wrote:
> > > To summarize, for vIOMMU we can work with the spec owner to
> > > define a proper interface to feedback such restriction into the guest
> > > if necessary. For the kernel part, it's clear that IOMMU fd should
> > > disallow two devices attached to a single [RID] or [RID, PASID] slot
> > > in the first place.
> > >
> > > Then the next question is how to communicate such restriction
> > > to the userspace. It sounds like a group, but different in concept.
> > > An iommu group describes the minimal isolation boundary thus all
> > > devices in the group can be only assigned to a single user. But this
> > > case is opposite - the two mdevs (both support ENQCMD submission)
> > > with the same parent have problem when assigned to a single VM
> > > (in this case vPASID is vm-wide translated thus a same pPASID will be
> > > used cross both mdevs) while they instead work pretty well when
> > > assigned to different VMs (completely different vPASID spaces thus
> > > different pPASIDs).
> > >
> > > One thought is to have vfio device driver deal with it. In this proposal
> > > it is the vfio device driver to define the PASID virtualization policy and
> > > report it to userspace via VFIO_DEVICE_GET_INFO. The driver understands
> > > the restriction thus could just hide the vPASID capability when the user
> > > calls GET_INFO on the 2nd mdev in above scenario. In this way the
> > > user even doesn't need to know such restriction at all and both mdevs
> > > can be assigned to a single VM w/o any problem.
> > >
> >
> > The restriction only probably happens when two mdevs are assigned to one
> > VM,
> > how could the vfio device driver get to know this info to accurately hide
> > the vPASID capability for the 2nd mdev when VFIO_DEVICE_GET_INFO?
> > There is no
> > need to do this in other cases.
> >
>
> I suppose the driver can detect it via whether two mdevs are opened by a
> single process.

Just have the kernel some ID for the PASID numberspace - devices with
the same ID have to be represented as a single RID.

Jason