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

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Message-ID: <ZuFB06X7ZTg6ZhWT@nvidia.com>
Date: Wed, 11 Sep 2024 00:08:03 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: "Tian, Kevin" <kevin.tian@...el.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)

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?

> > 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
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.

Thanks
Nicolin