Re: [PATCH v2 1/4] mm: Introduce vm_uffd_ops API

David Hildenbrand
2025-07-04T09:34:21Z

On 03.07.25 19:48, Mike Rapoport wrote:
> On Wed, Jul 02, 2025 at 03:46:57PM -0400, Peter Xu wrote:
>> On Wed, Jul 02, 2025 at 08:39:32PM +0300, Mike Rapoport wrote:
>>
>> [...]
>>
>>>> The main target of this change is the implementation of UFFD for
>>>> KVM/guest_memfd (examples: [1], [2]) to avoid bringing KVM-specific code
>>>> into the mm codebase. We usually mean KVM by the "drivers" in this context,
>>>> and it is already somewhat "knowledgeable" of the mm. I don't think there
>>>> are existing use cases for other drivers to implement this at the moment.
>>>>
>>>> Although I can't see new exports in this series, there is now a way to limit
>>>> exports to particular modules [3]. Would it help if we only do it for KVM
>>>> initially (if/when actually needed)?
>>>
>>> There were talks about pulling out guest_memfd core into mm, but I don't
>>> remember patches about it. If parts of guest_memfd were already in mm/ that
>>> would make easier to export uffd ops to it.
>>
>> Do we have a link to such discussion? I'm also curious whether that idea
>> was acknowledged by KVM maintainers.
>
> AFAIR it was discussed at one of David's guest_memfd calls

While it was discussed in the call a couple of times in different
context (guest_memfd as a library / guest_memfd shim), I think we
already discussed it back at LPC last year.

One of the main reasons for doing that is supporting guest_memfd in
other hypervisors -- the gunyah hypervisor in the kernel wants to make
use of it as well.

--
Cheers,

David / dhildenb