Re: [PATCH v2 00/19] iommufd: Add VIOMMU infrastructure (Part-1)

[Nicolin Chen <nicolinc@nvidia.com>]

On Fri, Sep 27, 2024 at 01:54:45PM +0800, Yi Liu wrote:
> > > > Baolu told me that Intel may have the same: different domain IDs
> > > > on different IOMMUs; multiple IOMMU instances on one chip:
> > > > https://lore.kernel.org/linux-iommu/cf4fe15c-8bcb-4132-a1fd-b2c8ddf2731b@linux.intel.com/
> > > > So, I think we are having the same situation here.
> > > 
> > > yes, it's called iommu unit or dmar. A typical Intel server can have
> > > multiple iommu units. But like Baolu mentioned in that thread, the intel
> > > iommu driver maintains separate domain ID spaces for iommu units, which
> > > means a given iommu domain has different DIDs when associated with
> > > different iommu units. So intel side is not suffering from this so far.
> > 
> > An ARM SMMU has its own VMID pool as well. The suffering comes
> > from associating VMIDs to one shared parent S2 domain.
> 
> Is this because of the VMID is tied with a S2 domain?

On ARM, yes. VMID is a part of S2 domain stuff.

> > Does a DID per S1 nested domain or parent S2? If it is per S2,
> > I think the same suffering applies when we share the S2 across
> > IOMMU instances?
> 
> per S1 I think. The iotlb efficiency is low as S2 caches would be
> tagged with different DIDs even the page table is the same. :)

On ARM, the stage-1 is tagged with an ASID (Address Space ID)
while the stage-2 is tagged with a VMID. Then an invalidation
for a nested S1 domain must require the VMID from the S2. The
ASID may be also required if the invalidation is specific to
that address space (otherwise, broadcast per VMID.)

I feel these two might act somehow similarly to the two DIDs
during nested translations?

> > > > Adding another vIOMMU wrapper on the other hand can allow us to
> > > > allocate different VMIDs/DIDs for different IOMMUs.
> > > 
> > > that looks like to generalize the association of the iommu domain and the
> > > iommu units?
> > 
> > A vIOMMU is a presentation/object of a physical IOMMU instance
> > in a VM.
> 
> a slice of a physical IOMMU. is it?

Yes. When multiple nested translations happen at the same time,
IOMMU (just like a CPU) is shared by these slices. And so is an
invalidation queue executing multiple requests.

Perhaps calling it a slice sounds more accurate, as I guess all
the confusion comes from the name "vIOMMU" that might be thought
to be a user space object/instance that likely holds all virtual
stuff like stage-1 HWPT or so?

> and you treat S2 hwpt as a resource of the physical IOMMU as well.

Yes. A parent HWPT (in the old day, we called it "kernel-manged"
HWPT) is not a user space thing. This belongs to a kernel owned
object.

> > This presentation gives a VMM some capability to take
> > advantage of some of HW resource of the physical IOMMU:
> > - a VMID is a small HW reousrce to tag the cache;
> > - a vIOMMU invalidation allows to access device cache that's
> >    not straightforwardly done via an S1 HWPT invalidation;
> > - a virtual device presentation of a physical device in a VM,
> >    related to the vIOMMU in the VM, which contains some VM-level
> >    info: virtual device ID, security level (ARM CCA), and etc;
> > - Non-PRI IRQ forwarding to the guest VM;
> > - HW-accelerated virtualization resource: vCMDQ, AMD VIOMMU;
> 
> might be helpful to draw a diagram to show what the vIOMMU obj contains.:)

That's what I plan to. Basically looks like:
  device---->stage1--->[ viommu [s2_hwpt, vmid, virq, HW-acc, etc.] ]

Thanks
Nic