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

[Jean-Philippe Brucker]

On 13/09/2018 17:55, Raj, Ashok wrote:
>> 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.

Right, for us assigning a PASID-granular mdev to a guest requires shadowing

>> 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.

I don't think the nesting attribute is described in details anywhere.
The SMMU drivers use it to know if they should create first- or
second-level mappings. At the moment QEMU always uses
VFIO_TYPE1v2_IOMMU, but Eric Auger is proposing a patch that adds
VFIO_TYPE1_NESTING_IOMMU to QEMU:
https://www.mail-archive.com/qemu-devel@xxxxxxxxxx/msg559820.html

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

Which case are you talking about? It might be more confusing than
helpful, but here's my understanding of what we can assign to a guest:

              |  no vIOMMU  | vIOMMU no PASID  | vIOMMU with PASID
--------------+-------------+------------------+--------------------
 VF           |     ok      |  shadow or nest  |      nest
 mdev, SMMUv3 |     ok      |     shadow       |  shadow + PV (?)
 mdev, vt-d3  |     ok      |      nest        |    nest + PV

The first line, assigning a PCI VF to a guest is the "basic" vfio-pci
case. Currently in QEMU it works by shadowing first-level translation.
We still have to upstream nested translation for that case. Vt-d2 didn't
support nested without PASID, vt-d3 offers RID_PASID for this. On SMMUv3
the PASID table is assigned to the guest, whereas on vt-d3 the host
manages the PASID table and individual page tables are assigned to the
guest.

Assigning an mdev (here I'm talking about the PASID-granular partition
of a VF, not the whole RID-granular VF wrapped by an mdev) could be done
by shadowing first-level translation on SMMUv3. It cannot do nested
since the VF has a single set of second-level page tables, which cannot
be used when mdevs are assigned to different VMs. Vt-d3 has one set of
second-level page tables per PASID, so it can do nested.

Since the parent device has a single PASID space, allowing the guest to
use multiple PASIDs for one mdev requires paravirtual allocation of
PASIDs (last column). Vt-d3 uses the Virtual Command Registers for that.
I assume that it is safe because the host is in charge of programming
PASIDs in the parent device, so the guest couldn't use a PASID allocated
to another mdev, but I don't know what the device's programming model
would look like. Anyway I don't think guest PASID is tackled by this
series (right?) and I don't intend to work on it for SMMUv3 (shadowing
stage-1 for vSVA seems like a bad idea...)

Does this seem accurate?

Thanks,
Jean