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

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Message-ID: <ZuFJh8t62LKi3FJY@nvidia.com>
Date: Wed, 11 Sep 2024 00:40:55 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
CC: "jgg@...dia.com" <jgg@...dia.com>, "will@...nel.org" <will@...nel.org>,
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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)

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

Yea, I can do that.

> > > > 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.

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