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

Nicolin Chen
2024-09-11T20:21:52Z

On Wed, Sep 11, 2024 at 08:08:04AM +0000, Tian, Kevin wrote:
> > From: Nicolin Chen <nicolinc@nvidia.com>
> > Sent: Wednesday, September 11, 2024 3:41 PM
> >
> > On Wed, Sep 11, 2024 at 07:18:10AM +0000, Tian, Kevin wrote:
> > > > From: Nicolin Chen <nicolinc@nvidia.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@nvidia.com>
> > > > > > Sent: Wednesday, August 28, 2024 1:00 AM
> > > > > >
> > > > > > 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 :)
> >
>
> I meant the 16bit VMID pool under each SMMU.

I see. Makes sense now.

Thanks
Nicolin