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

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Jason Gunthorpe [mailto:jgg@xxxxxxxxxx]
> Sent: 09 March 2023 15:40
> 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 03:26:12PM +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