Re: [PATCH RFCv1 04/14] iommufd: Add struct iommufd_viommu and iommufd_viommu_ops

Jason Gunthorpe
2024-05-24T13:03:39Z

On Fri, May 24, 2024 at 02:16:34AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Thursday, May 23, 2024 8:59 PM
> > On Thu, May 23, 2024 at 01:43:45AM +0000, Tian, Kevin wrote:
> > > > From: Jason Gunthorpe <jgg@nvidia.com>
> > > > Sent: Wednesday, May 22, 2024 9:39 PM
> > > >
> > > > The driver will then know it should program three different VMIDs for
> > > > the same S2 page table, which matches the ARM expectation for
> > > > VMID. That is to say we'd pass in the viommu as the pt_id for the
> > > > iommu_hwpt_alloc. The viommu would imply both the S2 page table and
> > > > any meta information like VMID the driver needs.
> > >
> > > Can you elaborate the aspect about "three different VMIDs"?
> >
> > In SMMUv3 the cache is tagged by (VMID,ASID) where ASID is completely
> > controlled by the guest.
> >
> > Every time the guest observes a SMMUv3 instance it is allowed to
> > creates its own private ASID number space for that instance. The guest
> > could re-use ASID #1 on two instances.
> >
> > So every SMMUv3 instance plugged into the guest needs to have its own
> > unique VMID so that the overlapping ASID's are disambiguate. The above
> > would create a VM where:
> >
> > deva -> vSMMUv3 #1
> > devb -> vSMMUv3 #2
> > devc -> No IOMMU
>
> This assumes that each vSMMUv3 instance has only one ASID space
> i.e. the guest cannot create multiple VMID's itself?

Right, the vSMMUv3 in the guest has no support VMID at all.

> > I assume the vPPRLog will steal all the PRI before it reaches the
> > kernel, so once this is turned on Lu's path won't see anything.
>
> Okay, then we expect this vqueue object only for HW acceleration
> while software-based fault logging is still routed via Baolu's work.

Yeah, I think it mirrors the invalidation that way, we have a SW
invalidation path and a HW path. The PRI is standards based so it is
easier to make a generic API for it.

Jason