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Date:   Mon, 11 Dec 2023 13:27:49 -0800
From:   Nicolin Chen <nicolinc@nvidia.com>
To:     Jason Gunthorpe <jgg@nvidia.com>
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Subject: Re: [PATCH v6 0/6] iommufd: Add nesting infrastructure (part 2/2)
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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?

> To make this work the iommu needs to be programmed with:
>  AMD: A vDomain-ID -> pDomain-ID table
>       A vRID -> pRID table
>       This is all bound to some "virtual function"
>  ARM: A vRID -> pRID table
>       The vCMDQ is bound to a VM_ID, so to the Nesting Parent

VCMDQ also has something called "virtual interface" that holds
a VMID and a list of CMDQ queues, which might be a bit similar
to AMD's "virtual function".

> For AMD, as above, I suggest the vDomain-ID be passed when creating
> the nesting domain.
> 
> The AMD "virtual function".. It is probably best to create a new iommufd
> object for this and it can be passed in to a few places
> 
> The vRID->pRID table should be some mostly common
> IOMMUFD_DEV_ASSIGN_VIRTUAL_ID. AMD will need to pass in the virtual
> function ID and ARM will need to pass in the Nesting Parent ID.

It sounds like our previous IOMMUFD_SET/UNSET_IDEV_DATA. I'm
wondering if we need to make it exclusive to the ID assigning?
Maybe set_idev_data could be reused for other potential cases?

If we do implement an IOMMUFD_DEV_ASSIGN_VIRTUAL_ID, do we need
an IOMMUFD_DEV_RESIGN_VIRTUAL_ID? (or better word than resign).

Could the structure just look like this?
struct iommu_dev_assign_virtual_id {
       __u32 size;
       __u32 dev_id;
       __u32 id_type;
       __u32 id;
};

> In many ways the nesting parent/virtual function are very similar
> things. Perhaps ARM should also create a virtual function object which
> is just welded to the nesting parent for API consistency.

A virtual function that holds an S2 domain/iopt + a VMID? If
this is for VCMDQ, the VMCDQ extension driver has that kinda
object holding an S2 domain: I implemented as the extension
function at the end of arm_smmu_finalise_s2() previously.

> So.. In short.. Invalidation is a PITA. The idea is the same but
> annoying little details interfere with actually having a compltely
> common API here. IMHO the uAPI in this series is fine. It will support
> Intel invalidation and non-ATC invalidation on AMD/ARM. It should be
> setup to allow that the target domain object can be any HWPT.
> 
> ARM will be able to do IOTLB invalidation using this API.
> 
> 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.

> Then in yet another series we can tackle the entire "virtual function"
> vRID/pRID translation stuff when the mmapable queue thing is
> introduced.

VCMDQ is also a mmapable queue. I feel that there could be
more common stuff between "virtual function" and "virtual
interface", I'll need to take a look at AMD's stuff though.

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
+};

> Thus next steps:
>  - Get an ARM patch that just does IOTLB invalidation and add it to my
>    part 3
>  - Start working on IOMMUFD_DEV_INVALIDATE along with an ARM
>    implementation of it.

I will work on these two, presumably including the new
IOMMUFD_DEV_ASSIGN_VIRTUAL_ID or so.

Thanks
Nicolin