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

On Wed, Sep 11, 2024 at 07:18:10AM +0000, Tian, Kevin wrote:

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From: Nicolin Chen <redacted>
Sent: Wednesday, September 11, 2024 3:08 PM

On Wed, Sep 11, 2024 at 06:12:21AM +0000, Tian, Kevin wrote:

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From: Nicolin Chen <redacted>
Sent: Wednesday, August 28, 2024 1:00 AM

[...]

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On a multi-IOMMU system, the VIOMMU object can be instanced to the
number
of vIOMMUs in a guest VM, while holding the same parent HWPT to

share

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the

Is there restriction that multiple vIOMMU objects can be only created
on a multi-IOMMU system?

I think it should be generally restricted to the number of pIOMMUs,
although likely (not 100% sure) we could do multiple vIOMMUs on a
single-pIOMMU system. Any reason for doing that?

No idea. But if you stated so then there will be code to enforce it e.g.
failing the attempt to create a vIOMMU object on a pIOMMU to which
another vIOMMU object is already linked?

Yea, I can do that.

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stage-2 IO pagetable. Each VIOMMU then just need to only allocate its

own

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VMID to attach the shared stage-2 IO pagetable to the physical IOMMU:

this reads like 'VMID' is a virtual ID allocated by vIOMMU. But from the
entire context it actually means the physical 'VMID' allocated on the
associated physical IOMMU, correct?

Quoting Jason's narratives, a VMID is a "Security namespace for
guest owned ID". The allocation, using SMMU as an example, should

the VMID alone is not a namespace. It's one ID to tag another namespace.

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be a part of vIOMMU instance allocation in the host SMMU driver.
Then, this VMID will be used to mark the cache tags. So, it is
still a software allocated ID, while HW would use it too.

VMIDs are physical resource belonging to the host SMMU driver.

Yes. Just the lifecycle of a VMID is controlled by a vIOMMU, i.e.
the guest.

but I got your original point that it's each vIOMMU gets an unique VMID
from the host SMMU driver, not exactly that each vIOMMU maintains
its own VMID namespace. that'd be a different concept.

What's a VMID namespace actually? Please educate me :)

Thanks
Nicolin