Re: [RFC PATCH v2 00/10] vfio/mdev: IOMMU aware mediated device

["Raj, Ashok" <ashok.raj@intel.com>]

On Thu, Sep 13, 2018 at 04:03:01PM +0100, Jean-Philippe Brucker wrote:
> On 13/09/2018 01:19, Tian, Kevin wrote:
> >>> This is proposed for architectures which support finer granularity
> >>> second level translation with no impact on architectures which only
> >>> support Source ID or the similar granularity.
> >>
> >> Just to be clear, in this paragraph you're only referring to the
> >> Nested/second-level translation for mdev, which is specific to vt-d
> >> rev3? Other architectures can still do first-level translation with
> >> PASID, to support some use-cases of IOMMU aware mediated device
> >> (assigning mdevs to userspace drivers, for example)
> > 
> > yes. aux domain concept applies only to vt-d rev3 which introduces
> > scalable mode. Care is taken to avoid breaking usages on existing
> > architectures.
> > 
> > one note. Assigning mdevs to user space alone doesn't imply IOMMU
> > aware. All existing mdev usages use software or proprietary methods to
> > isolate DMA. There is only one potential IOMMU aware mdev usage 
> > which we talked not rely on vt-d rev3 scalable mode - wrap a random 
> > PCI device into a single mdev instance (no sharing). In that case mdev 
> > inherits RID from parent PCI device, thus is isolated by IOMMU in RID 
> > granular. Our RFC supports this usage too. In VFIO two usages (PASID-
> > based and RID-based) use same code path, i.e. always binding domain to
> > the parent device of mdev. But within IOMMU they go different paths.
> > PASID-based will go to aux-domain as iommu_enable_aux_domain
> > has been called on that device. RID-based will follow existing 
> > unmanaged domain path, as if it is parent device assignment.
> 
> For Arm SMMU we're more interested in the PASID-granular case than the
> RID-granular one. It doesn't necessarily require vt-d rev3 scalable
> mode, the following example can be implemented with an SMMUv3, since it
> only needs PASID-granular first-level translation:

You are right, you can simply use the first level as IOVA for every PASID.

Only issue becomes when you need to assign that to a guest, you would be required
to shadow the 1st level. If you have a 2nd level per-pasid first level can
be managed in guest and don't require to shadow them.

> 
> We have a PCI function that supports PASID, and can be partitioned into
> multiple isolated entities, mdevs. Each mdev has an MMIO frame, an MSI
> vector and a PASID.
> 
> Different processes (userspace drivers, not QEMU) each open one mdev. A
> process controlling one mdev has two ways of doing DMA:
> 
> (1) Classically, the process uses a VFIO_TYPE1v2_IOMMU container. This
> creates an auxiliary domain for the mdev, with PASID #35. The process
> creates DMA mappings with VFIO_IOMMU_MAP_DMA. VFIO calls iommu_map on
> the auxiliary domain. The IOMMU driver populates the pgtables associated
> with PASID #35.
> 
> (2) SVA. One way of doing it: the process uses a new
> "VFIO_TYPE1_SVA_IOMMU" type of container. VFIO binds the process address
> space to the device, gets PASID #35. Simpler, but not everyone wants to
> use SVA, especially not userspace drivers which need the highest
> performance.
> 
> 
> This example only needs to modify first-level translation, and works
> with SMMUv3. The kernel here could be the host, in which case
> second-level translation is disabled in the SMMU, or it could be the
> guest, in which case second-level mappings are created by QEMU and
> first-level translation is managed by assigning PASID tables to the guest.
> 
> So (2) would use iommu_sva_bind_device(), but (1) needs something else.
> Aren't auxiliary domains suitable for (1)? Why limit auxiliary domain to
> second-level or nested translation? It seems silly to use a different
> API for first-level, since the flow in userspace and VFIO is the same as
> your second-level case as far as MAP_DMA ioctl goes. The difference is
> that in your case the auxiliary domain supports an additional operation
> which binds first-level page tables. An auxiliary domain that only
> supports first-level wouldn't support this operation, but it can still
> implement iommu_map/unmap/etc.
> 
> 
> Another note: if for some reason you did want to allow userspace to
> choose between first-level or second-level, you could implement the
> VFIO_TYPE1_NESTING_IOMMU container. It acts like a VFIO_TYPE1v2_IOMMU,
> but also sets the DOMAIN_ATTR_NESTING on the IOMMU domain. So DMA_MAP
> ioctl on a NESTING container would populate second-level, and DMA_MAP on
> a normal container populates first-level. But if you're always going to
> use second-level by default, the distinction isn't necessary.

Where is the nesting attribute specified? in vt-d2 it was part of context
entry, so also meant all PASID's are nested now. In vt-d3 its part of
PASID context.

It seems unsafe to share PASID's with different VM's since any request 
W/O PASID has only one mapping.

> 
> 
> >> Sounds good, I'll drop the private PASID patch if we can figure out a
> >> solution to the attach/detach_dev problem discussed on patch 8/10
> >>
> > 
> > Can you elaborate a bit on private PASID usage? what is the
> > high level flow on it? 
> > 
> > Again based on earlier explanation, aux domain is specific to IOMMU
> > architecture supporting vtd scalable mode-like capability, which allows
> > separate 2nd/1st level translations per PASID. Need a better understanding
> > how private PASID is relevant here.
> 
> Private PASIDs are used for doing iommu_map/iommu_unmap on PASIDs
> (first-level translation):
> https://www.spinics.net/lists/dri-devel/msg177003.html As above, some
> people don't want SVA, some can't do it, some may even want a few
> private address spaces just for their kernel driver. They need a way to
> allocate PASIDs and do iommu_map/iommu_unmap on them, without binding to
> a process. I was planning to add the private PASID patch to my SVA
> series, but in my opinion the feature overlaps with auxiliary domains.

It sounds like it maps to AUX domains.