Re: [PATCH v1 02/14] iommufd: Add nesting related data structures for ARM SMMUv3

[Nicolin Chen]

On Thu, Mar 09, 2023 at 04:07:54PM +0000, Shameerali Kolothum Thodi wrote:
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> > -----Original Message-----
> > From: Jason Gunthorpe [mailto:jgg@xxxxxxxxxx]
> > Sent: 09 March 2023 16:00
> > To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@xxxxxxxxxx>
> > Cc: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>; Nicolin Chen
> > <nicolinc@xxxxxxxxxx>; robin.murphy@xxxxxxx; will@xxxxxxxxxx;
> > eric.auger@xxxxxxxxxx; kevin.tian@xxxxxxxxx; baolu.lu@xxxxxxxxxxxxxxx;
> > joro@xxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
> > iommu@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; yi.l.liu@xxxxxxxxx
> > Subject: Re: [PATCH v1 02/14] iommufd: Add nesting related data structures
> > for ARM SMMUv3
> >
> > On Thu, Mar 09, 2023 at 03:51:42PM +0000, Shameerali Kolothum Thodi
> > wrote:
> >
> > > > For ARM cases where there is no shared VMID space with KVM, the ARM
> > > > VMID should be somehow assigned to the iommfd_ctx itself and the alloc
> > > > domain op should receive it from there.
> > >
> > > Is there any use of VMID outside SMMUv3? I was thinking if nested domain
> > alloc
> > > doesn't provide the KVM instance, then SMMUv3 can use its internal VMID.
> >
> > When we talk about exposing an SMMUv3 IOMMU CMDQ directly to
> > userspace then
> > VMID is the security token that protects it.
> >
> > So in that environment every domain under the same iommufd should
> > share the same VMID so that the CMDQ's also share the same VMID.
> >
> > I expect this to be a common sort of requirement as we will see
> > userspace command queues in the other HW as well.
> >
> > So, I suppose the answer for now is that ARM SMMUv3 should just
> > allocate one VMID per iommu_domain and there should be no VMID in the
> > uapi at all.
> >
> > Moving all iommu_domains to share the same VMID is a future patch.
> >
> > Though.. I have no idea how vVMID is handled in the SMMUv3
> > architecture. I suppose the guest IOMMU HW caps are set in a way that
> > it knows it does not have VMID?
> 
> I think, Guest only sets up the SMMUv3 S1 stage and it doesn't use VMID.

Yea, a vmid is only allocated in an S2 domain allocation. So,
a guest allocating only S1 domains always sets VMID=0. Yet, I
think that the hypervisor or some where in host kernel should
replace the VMID=0 with a unified VMID.

Thanks
Nic