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

[Nicolin Chen <nicolinc@nvidia.com>]

On Thu, Mar 09, 2023 at 09:26:57PM -0800, Nicolin Chen wrote:
> On Thu, Mar 09, 2023 at 04:07:54PM +0000, Shameerali Kolothum Thodi wrote:
> > External email: Use caution opening links or attachments
> > 
> > 
> > > -----Original Message-----
> > > From: Jason Gunthorpe [mailto:jgg@nvidia.com]
> > > Sent: 09 March 2023 16:00
> > > To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>
> > > Cc: Jean-Philippe Brucker <jean-philippe@linaro.org>; Nicolin Chen
> > > <nicolinc@nvidia.com>; robin.murphy@arm.com; will@kernel.org;
> > > eric.auger@redhat.com; kevin.tian@intel.com; baolu.lu@linux.intel.com;
> > > joro@8bytes.org; linux-arm-kernel@lists.infradead.org;
> > > iommu@lists.linux.dev; linux-kernel@vger.kernel.org; yi.l.liu@intel.com
> > > 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.

Ah, I just recall a conversation with Jason that a VM should only
have one S2 domain. In that case, the VMID is already unified?

Thanks
Nic