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

[Nicolin Chen <nicolinc@nvidia.com>]

On Sun, May 12, 2024 at 11:03:53AM -0300, Jason Gunthorpe wrote:
> On Fri, Apr 12, 2024 at 08:47:01PM -0700, Nicolin Chen wrote:
> > Add a new iommufd_viommu core structure to represent a vIOMMU instance in
> > the user space, typically backed by a HW-accelerated feature of an IOMMU,
> > e.g. NVIDIA CMDQ-Virtualization (an ARM SMMUv3 extension) and AMD Hardware
> > Accelerated Virtualized IOMMU (vIOMMU).
> 
> I expect this will also be the only way to pass in an associated KVM,
> userspace would supply the kvm when creating the viommu.
> 
> The tricky bit of this flow is how to manage the S2. It is necessary
> that the S2 be linked to the viommu:
> 
>  1) ARM BTM requires the VMID to be shared with KVM
>  2) AMD and others need the S2 translation because some of the HW
>     acceleration is done inside the guest address space
> 
> I haven't looked closely at AMD but presumably the VIOMMU create will
> have to install the S2 into a DID or something?
> 
> So we need the S2 to exist before the VIOMMU is created, but the
> drivers are going to need some more fixing before that will fully
> work.
> 
> Does the nesting domain create need the viommu as well (in place of
> the S2 hwpt)? That feels sort of natural.

Yes, I had a similar thought initially: each viommu is backed by
a nested IOMMU HW, and a special HW accelerator like VCMDQ could
be treated as an extension on top of that. It might not be very
straightforward like the current design having vintf<->viommu and
vcmdq <-> vqueue though...

In that case, we can then support viommu_cache_invalidate, which
is quite natural for SMMUv3. Yet, I recall Kevin said that VT-d
doesn't want or need that.

> There is still a lot of fixing before everything can work fully, but
> do we need to make some preperations here in the uapi? Like starting
> to thread the S2 through it as I described?
> 
> Kevin, does Intel forsee any viommu needs on current/future Intel HW?
> I assume you are thinking about invalidation queue bypass like
> everyone else. I think it is an essential feature for vSVA.
> 
> > A driver should embed this core structure in its driver viommu structure
> > and call the new iommufd_viommu_alloc() helper to allocate a core/driver
> > structure bundle and fill its core viommu->ops:
> >     struct my_driver_viommu {
> >         struct iommufd_viommu core;
> > 	....
> >     };
> > 
> >     static const struct iommufd_viommu_ops my_driver_viommu_ops = {
> >         .free = my_driver_viommu_free,
> >     };
> > 
> >     struct my_driver_viommu *my_viommu =
> >             iommufd_viommu_alloc(my_driver_viommu, core);
> 
> Why don't we have an ictx here anyhow? The caller has it? Just pass it
> down and then it is normal:
> 
> my_viommu = iommufd_object_alloc_elm(ictx, my_viommu, IOMMUFD_OBJ_HWPT_VIOMMU, core.obj);

Oh, in that case, we probably don't need a level-3 obj allocator
that was previously missing ictx to allocate an obj->id.

Thanks
Nicolin