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

[Nicolin Chen]

On Thu, Mar 09, 2023 at 11:40:16AM -0400, Jason Gunthorpe wrote:
> 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.
> 
> If there is no KVM it should either fail to setup the domain or setup
> a domain disconnected from KVM.
> 
> 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.
> 
> Nicolin, that seems to be missing in this series? I'm not entirely
> sure how to elegantly code it :\

Yea, it's missing. The VMID thing is supposed to be a sneak peek
of my next VCMDQ solution. Now it seems that BTM needs this too.

Remember that my previous VCMDQ series had a big complication to
share VMID across the passthrough devices in the same VM? During
that patch review, we concluded that IOMMUFD would simply align
VMIDs using a unified ctx ID or so, IIRC.

Thanks
Nic