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

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Message-ID: <BN9PR11MB5276554A016C2A54C41C64808C9B2@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Wed, 11 Sep 2024 07:18:10 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Nicolin Chen <nicolinc@...dia.com>
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	<smostafa@...gle.com>, "Liu, Yi L" <yi.l.liu@...el.com>
Subject: RE: [PATCH v2 00/19] iommufd: Add VIOMMU infrastructure (Part-1)

> From: Nicolin Chen <nicolinc@...dia.com>
> Sent: Wednesday, September 11, 2024 3:08 PM
> 
> On Wed, Sep 11, 2024 at 06:12:21AM +0000, Tian, Kevin wrote:
> > > From: Nicolin Chen <nicolinc@...dia.com>
> > > Sent: Wednesday, August 28, 2024 1:00 AM
> > >
> > [...]
> > > 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
> > > 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?

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

> 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.

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.