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

Shameerali Kolothum Thodi
2023-03-09T15:51:52Z

> -----Original Message-----
> From: Jason Gunthorpe [mailto:jgg@nvidia.com]
> Sent: 09 March 2023 15:40
> 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 03:26:12PM +0000, Shameerali Kolothum Thodi
> wrote:
>
> > On platforms that supports BTM [1], we may need the VMID allocated by
> KVM.
> > But again getting that from user pace doesn't look safe. I have attempted
> to revise
> > the earlier RFC to pin and use the KVM VMID from SMMUv3 here[2].
>
> Gurk
>
> > " In the new design we can require from the start that creating a nesting
> IOMMU
> > container through /dev/iommu *must* come with a KVM context, that way
> > we're sure to reuse the existing VMID. "
>
> I've been dreading this but yes I execpt we will eventually need to
> connect kvm and iommufd together. The iommu driver can receive a kvm
> pointer as part of the alloc domain operation to do any setups like
> this.

That will make life easier :)

> If there is no KVM it should either fail to setup the domain or setup
> a domain disconnected from KVM.
>

If no KVM the SMMUv3 can fall back to its internal VMID allocation I guess.
And my intention was to use KVM VMID only if the platform supports
BTM.

> If IOMMU HW and KVM HW are using the same ID number space then
> arguably the two kernel drivers need to use a shared ID allocator in
> the arch, regardless of what iommufd/etc does. Using KVM should not be
> mandatory for iommufd.
>
> 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.

Thanks,
Shameer

> Nicolin, that seems to be missing in this series? I'm not entirely
> sure how to elegantly code it :\
>
> Jason