Re: [PATCH RFCv1 07/14] iommufd: Add viommu set/unset_dev_id ops

Nicolin Chen
2024-05-24T03:26:48Z

On Fri, May 24, 2024 at 02:21:23AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@nvidia.com>
> > Sent: Thursday, May 23, 2024 11:01 PM
> >
> > On Thu, May 23, 2024 at 06:19:59AM +0000, Tian, Kevin wrote:
> > > > From: Jason Gunthorpe <jgg@nvidia.com>
> > > > Sent: Wednesday, May 22, 2024 2:25 AM
> > > >
> > > > On Tue, May 14, 2024 at 06:59:07PM -0700, Nicolin Chen wrote:
> > > > > So, you want a proxy S1 domain for a device to attach, in case
> > > > > of a stage-2 only setup, because an S2 domain will no longer has
> > > > > a VMID, since it's shared among viommus. In the SMMU driver case,
> > > > > an arm_smmu_domain won't have an smmu pointer, so a device can't
> > > > > attach to an S2 domain but always an nested S1 domain, right?
> > > >
> > > > That seems like a simple solution to the VMID lifetime, but it means
> > > > the kernel has to decode more types of vSTE.
> > > >
> > >
> > > why does ATC invalidation need to know about VMID?
> >
> > ATC invalidation always requires a vRID to pRID translation and the
> > VIOMMU will hold that translation.
> >
> > On vCMDQ HW and on AMD HW the vRID to pRID translation is pushed into
> > HW, and vCMDQ requires the VMID to do that.
> >
>
> At a quick glance VMID and vRID->pRID translation are both configurations
> of a vintf.
>
> My impression was that vintf->vmid is added to guest cmd when it's
> about iotlb invalidation.
>
> then vintf->sid_slots is walked when handling a guest cmd for ATC
> invalidation.
>
> I'm not sure why the latter one requires a valid VMID to do the walking
> except it's a implementation choice made by vCMDQ?

Well, we haven't thought about a case of doing ATC invalidation
via VINTF/VCMDQ without setting up a VMID, as "VMID" is a field
in the VINTF_CONFIG register next to the Enable bit and must be
set prior to enabling a VINTF, though your understanding of the
HW work flow is probably accurate :)

And the narrative at the top was trying to describe the links:
[ device ] => [ proxy identity S1 ] => [ viommu [ shared S2 ] ]
v.s.
[ device ] => [ non-shareable S2 ]

So the first case can take advantage of VIOMMU_INVALIDATE v.s.
the second case requires a DEV_INVALIDATE.

Another conclusion between the lines: since ARM SMMU will have
the first case (with viommu), there is no longer any use of a
DEV_INVALIDATE ioctl. So, we would likely drop it in the coming
version.

Thanks
Nicolin