Re: [PATCH v6 0/6] iommufd: Add nesting infrastructure (part 2/2)

[Nicolin Chen]

On Mon, Dec 11, 2023 at 05:57:38PM -0400, Jason Gunthorpe wrote:
> On Mon, Dec 11, 2023 at 01:27:49PM -0800, Nicolin Chen wrote:
> > On Fri, Dec 08, 2023 at 09:47:26PM -0400, Jason Gunthorpe wrote:
> > > What is in a Nested domain:
> > >  ARM: A CD table pointer
> > >       Nesting domains are created for every unique CD table top pointer.
> > 
> > I think we basically implemented in a way of syncing STE, i,e,
> > vSTE.Config must be "S1 Translate" besides a CD table pointer,
> > and a nested domain is freed when vSTE.Config=BYPASS even if a
> > CD table pointer is present, right?
> 
> Yes, but you can also de-duplicate the nested domains based on the CD
> table pointer. It is not as critical for ARM as others, but may
> still be worth doing.

Ah right! Should do that.

> > If we do implement an IOMMUFD_DEV_ASSIGN_VIRTUAL_ID, do we need
> > an IOMMUFD_DEV_RESIGN_VIRTUAL_ID? (or better word than resign).
> 
> I don't think so.. The vRID is basically fixed, if it needs to be
> changed then the device can be destroyed (or assign can just change it)

OK. Will insert the cleanup piece to the unbind()/destroy().

> > Could the structure just look like this?
> > struct iommu_dev_assign_virtual_id {
> >        __u32 size;
> >        __u32 dev_id;
> >        __u32 id_type;
> >        __u32 id;
> > };
> 
> It needs to take in the viommu_id also, and I'd make the id 64 bits
> just for good luck.

What is viommu_id required for in this context? I thought we
already know which SMMU instance to issue commands via dev_id?

> > > IOMMUFD_DEV_INVALIDATE should be introduced with the same design as
> > > HWPT invalidate. This would be used for AMD/ARM's ATC invalidation
> > > (and just force the stream ID, userspace must direct the vRID to the
> > > correct dev_id).
> > 
> > SMMU's CD invalidations could fall into this category too.

Do we need a different iommu API for this ioctl? We currently
have the cache_invalidate_user as a domain op. The new device
op will be an iommu op?

And should we rename the "cache_invalidate_user"? Would VT-d
still uses it for device cache?

> > I previously drafted something to test it out with iommufd.
> > Basically it needs the pairing of vRID/pRID in attach_dev()
> > and another ioctl to mmap/config user queue(s):
> > +struct iommu_hwpt_cache_config_tegra241_vcmdq {
> > +       __u32 vcmdq_id;			// queue id
> > +       __u32 vcmdq_log2size;		// queue size
> > +       __aligned_u64 vcmdq_base;	// queue guest PA
> > +};
> 
> vRID/pRID pairing should come from IOMMUFD_DEV_ASSIGN_VIRTUAL_ID. When
> a HWPT is allocated it would be connected to the viommu_id and then it
> would all be bundled together in the HW somehow

Since we were talking about sharing stage-2 domain, the HWPT
to the stage-2 domain will be shared among the vIOMMU/pIOMMU
instances too? I think I am not quite getting the viommu_id
part yet. From the other side of this thread, viommu object
is created per vIOMMU instance and each one actually tied to
a pIOMMU by nature?

Thanks
Nicolin