Re: [RFC PATCH v2 12/15] vfio: Support VFIO devices that access the CPU MMU

[Ryan Roberts <ryan.roberts@arm.com>]

On 20/10/2026 15:09, Jason Gunthorpe wrote:
> On Tue, Oct 20, 2026 at 12:51:26PM +0100, Ryan Roberts wrote:
>>> However.. you still have to affirm which single mm is the mm that the
>>> device is going to be doing DMA against. The process the opens the fd
>>> should not be forced to be the mm that it gets linked to.
>>
>> What is the mm representing here? Ultimately we want to install the KVM
>> instance's stage2 pgtables and (pinned) VMID. We currently get that via the
>> bound KVM instance (which looks like it still works using the cdev interface?)
>>
>> The guest will then install the stage1 pgtables and (pinned) ASID associated
>> with the relevant (guest) mm_struct.
>>
>> Do we _need_ an mm association in the host?
> 
> Ah.. I see, I didn't quite follow that. OK.. Ah everyone else doing
> this is using iommufd with a viommu to establish their connection
> between device an KVM. eg Jacob Pan's series. That's a probably a
> better idea to stay aligned with the common path. Though your reasons
> are a little different..

Just to check, you're referring to [1]?

I was under the impression that I could open the VFIO device through the cdev
interface, then bind it to the KVM instance via KVM_DEV_VFIO_FILE_ADD... Why do
I need an iommufd at all?

[1]
https://lore.kernel.org/all/20260925190742.1575380-1-jacob.pan@linux.microsoft.com/#t

Thanks,
Ryan

> 
> If you manage to get pinned VMID out of KVM then we can implement SMMU
> BTM as well!
> 
> Jason