This is a revival of the guestmem library patch series originated from
Elliot [1].  The reason I am bringing it up now is it would help
implement UserfaultFD support minor mode in guest_memfd.

Background

We are building a Firecracker version that uses guest_memfd to back
guest memory [2].  The main objective is to use guest_memfd to remove
guest memory from host kernel's direct map to reduce the surface for
Spectre-style transient execution issues [3].  Currently, Firecracker
supports restoring VMs from snapshots using UserfaultFD [4], which is
similar to the postcopy phase of live migration.  During restoration,
while we rely on a separate mechanism to handle stage-2 faults in
guest_memfd [5], UserfaultFD support in guest_memfd is still required to
handle faults caused either by the VMM itself or by MMIO access handling
on x86.

The major problem in implementing UserfaultFD for guest_memfd is that
the MM code (UserfaultFD) needs to call KVM-specific interfaces.
Particularly for the minor mode, these are 1) determining the type of
the VMA (eg is_vma_guest_memfd()) and 2) obtaining a folio (ie
kvm_gmem_get_folio()).  Those may not be always available as KVM can be
compiled as a module.  Peter attempted to approach it via exposing an
ops structure where modules (such as KVM) could provide their own
callbacks, but it was not deemed to be sufficiently safe as it opens up
an unrestricted interface for all modules and may leave MM in an
inconsistent state [6].

An alternative way to make these interfaces available to the UserfaultFD
code is extracting generic-MM guest_memfd parts into a library
(guestmem) under MM where they can be safely consumed by the UserfaultFD
code.  As far as I know, the original guestmem library series was
motivated by adding guest_memfd support in Gunyah hypervisor [7].

This RFC

I took Elliot's v5 (the latest) and rebased it on top of the guest_memfd
preview branch [8] because I also wanted to see how it would work with
direct map removal [3] and write syscall [9], which are building blocks
for the guest_memfd-based Firecracker version.  On top of it I added a
patch that implements UserfaultFD support for guest_memfd using
interfaces provided by the guestmem library to illustrate the complete
idea.

I made the following modifications along the way:
 - Followed by a comment from Sean, converted invalidate_begin()
   callback back to void as it cannot fail in KVM, and the related
   Gunyah requirement is unknown to me
 - Extended the guestmem_ops structure with the supports_mmap() callback
   to provide conditional mmap support in guestmem
 - Extended the guestmem library interface with guestmem_allocate(),
   guestmem_test_no_direct_map(), guestmem_mark_prepared(),
   guestmem_mmap(), and guestmem_vma_is_guestmem()
 - Made (kvm_gmem)/(guestmem)_test_no_direct_map() use
   mapping_no_direct_map() instead of KVM-specific flag
   GUEST_MEMFD_FLAG_NO_DIRECT_MAP to make it KVM-independent

Feedback that I would like to receive:
 - Is this the right solution to the "UserfaultFD in guest_memfd"
   problem?
 - What requirements from other hypervisors than KVM do we need to
   consider at this point?
 - Does the line between generic-MM and KVM-specific guest_memfd parts
   look sensible?

Previous iterations of UserfaultFD support in guest_memfd patches:
v3:
 - https://lore.kernel.org/kvm/20250404154352.23078-1-kalyazin@amazon.com
 - minor changes to address review comments (James)
v2:
 - https://lore.kernel.org/kvm/20250402160721.97596-1-kalyazin@amazon.com
 - implement a full minor trap instead of hybrid missing/minor trap
   (James/Peter)
 - make UFFDIO_CONTINUE implementation generic calling vm_ops->fault()
v1:
 - https://lore.kernel.org/kvm/20250303133011.44095-1-kalyazin@amazon.com

Nikita

[1]: https://lore.kernel.org/kvm/20241122-guestmem-library-v5-2-450e92951a15@quicinc.com
[2]: https://github.com/firecracker-microvm/firecracker/tree/feature/secret-hiding
[3]: https://lore.kernel.org/kvm/20250912091708.17502-1-roypat@amazon.co.uk
[4]: https://github.com/firecracker-microvm/firecracker/blob/main/docs/snapshotting/handling-page-faults-on-snapshot-resume.md
[5]: https://lore.kernel.org/kvm/20250618042424.330664-1-jthoughton@google.com
[6]: https://lore.kernel.org/linux-mm/20250627154655.2085903-1-peterx@redhat.com
[7]: https://lore.kernel.org/lkml/20240222-gunyah-v17-0-1e9da6763d38@quicinc.com
[8]: https://git.kernel.org/pub/scm/linux/kernel/git/david/linux.git/log/?h=guestmemfd-preview
[9]: https://lore.kernel.org/kvm/20250902111951.58315-1-kalyazin@amazon.com

Nikita Kalyazin (2):
  mm: guestmem: introduce guestmem library
  userfaulfd: add minor mode for guestmem

 Documentation/admin-guide/mm/userfaultfd.rst |   4 +-
 MAINTAINERS                                  |   2 +
 fs/userfaultfd.c                             |   3 +-
 include/linux/guestmem.h                     |  46 +++
 include/linux/userfaultfd_k.h                |   8 +-
 include/uapi/linux/userfaultfd.h             |   8 +-
 mm/Kconfig                                   |   3 +
 mm/Makefile                                  |   1 +
 mm/guestmem.c                                | 380 +++++++++++++++++++
 mm/userfaultfd.c                             |  14 +-
 virt/kvm/Kconfig                             |   1 +
 virt/kvm/guest_memfd.c                       | 303 ++-------------
 12 files changed, 493 insertions(+), 280 deletions(-)
 create mode 100644 include/linux/guestmem.h
 create mode 100644 mm/guestmem.c


base-commit: 911634bac3107b237dcd8fdcb6ac91a22741cbe7
-- 
2.50.1

Hello, Nikita,

On Mon, Sep 15, 2025 at 04:18:16PM +0000, Kalyazin, Nikita wrote:
> This is a revival of the guestmem library patch series originated from
> Elliot [1].  The reason I am bringing it up now is it would help
> implement UserfaultFD support minor mode in guest_memfd.
> 
> Background
> 
> We are building a Firecracker version that uses guest_memfd to back
> guest memory [2].  The main objective is to use guest_memfd to remove
> guest memory from host kernel's direct map to reduce the surface for
> Spectre-style transient execution issues [3].  Currently, Firecracker
> supports restoring VMs from snapshots using UserfaultFD [4], which is
> similar to the postcopy phase of live migration.  During restoration,
> while we rely on a separate mechanism to handle stage-2 faults in
> guest_memfd [5], UserfaultFD support in guest_memfd is still required to
> handle faults caused either by the VMM itself or by MMIO access handling
> on x86.
> 
> The major problem in implementing UserfaultFD for guest_memfd is that
> the MM code (UserfaultFD) needs to call KVM-specific interfaces.
> Particularly for the minor mode, these are 1) determining the type of
> the VMA (eg is_vma_guest_memfd()) and 2) obtaining a folio (ie
> kvm_gmem_get_folio()).  Those may not be always available as KVM can be
> compiled as a module.  Peter attempted to approach it via exposing an
> ops structure where modules (such as KVM) could provide their own
> callbacks, but it was not deemed to be sufficiently safe as it opens up
> an unrestricted interface for all modules and may leave MM in an
> inconsistent state [6].

I apologize when I was replying to your offlist email that I'll pick it up,
but I didn't.. I moved on with other things after the long off which was
more urgent, then I never got the chance to go back..  I will do it this
week.

I don't think it's a real safety issue.  Frankly, I still think that latest
patchset, as-is, is the best we should come up with userfaultfd.  If people
worry about uffdio_copy(), it's fine, we can drop it.  It's not a huge deal
at least for now.

Btw, thanks for help pinging that thread, and sorry I didn't yet get back
to it.  I'll read the discussions (I didn't yet, after back to work for
weeks), but I will.

> 
> An alternative way to make these interfaces available to the UserfaultFD
> code is extracting generic-MM guest_memfd parts into a library
> (guestmem) under MM where they can be safely consumed by the UserfaultFD
> code.  As far as I know, the original guestmem library series was
> motivated by adding guest_memfd support in Gunyah hypervisor [7].
> 
> This RFC
> 
> I took Elliot's v5 (the latest) and rebased it on top of the guest_memfd
> preview branch [8] because I also wanted to see how it would work with
> direct map removal [3] and write syscall [9], which are building blocks
> for the guest_memfd-based Firecracker version.  On top of it I added a
> patch that implements UserfaultFD support for guest_memfd using
> interfaces provided by the guestmem library to illustrate the complete
> idea.

I hope patch 2 exactly illustrated on why the uffd modulization effort is
still worthwhile (to not keep attaching "if"s all over the places).  Would
you agree?

If you agree, we'll need to review the library work as a separate effort
from userfaultfd.

> 
> I made the following modifications along the way:
>  - Followed by a comment from Sean, converted invalidate_begin()
>    callback back to void as it cannot fail in KVM, and the related
>    Gunyah requirement is unknown to me
>  - Extended the guestmem_ops structure with the supports_mmap() callback
>    to provide conditional mmap support in guestmem
>  - Extended the guestmem library interface with guestmem_allocate(),
>    guestmem_test_no_direct_map(), guestmem_mark_prepared(),
>    guestmem_mmap(), and guestmem_vma_is_guestmem()
>  - Made (kvm_gmem)/(guestmem)_test_no_direct_map() use
>    mapping_no_direct_map() instead of KVM-specific flag
>    GUEST_MEMFD_FLAG_NO_DIRECT_MAP to make it KVM-independent
> 
> Feedback that I would like to receive:
>  - Is this the right solution to the "UserfaultFD in guest_memfd"
>    problem?

Yes it's always a fair question to ask.  I shared my two cents above.  We
can definitely also hear about how others think.

I hope I'll keep my words this time on reposting.

Thanks,

>  - What requirements from other hypervisors than KVM do we need to
>    consider at this point?
>  - Does the line between generic-MM and KVM-specific guest_memfd parts
>    look sensible?
> 
> Previous iterations of UserfaultFD support in guest_memfd patches:
> v3:
>  - https://lore.kernel.org/kvm/20250404154352.23078-1-kalyazin@amazon.com
>  - minor changes to address review comments (James)
> v2:
>  - https://lore.kernel.org/kvm/20250402160721.97596-1-kalyazin@amazon.com
>  - implement a full minor trap instead of hybrid missing/minor trap
>    (James/Peter)
>  - make UFFDIO_CONTINUE implementation generic calling vm_ops->fault()
> v1:
>  - https://lore.kernel.org/kvm/20250303133011.44095-1-kalyazin@amazon.com
> 
> Nikita
> 
> [1]: https://lore.kernel.org/kvm/20241122-guestmem-library-v5-2-450e92951a15@quicinc.com
> [2]: https://github.com/firecracker-microvm/firecracker/tree/feature/secret-hiding
> [3]: https://lore.kernel.org/kvm/20250912091708.17502-1-roypat@amazon.co.uk
> [4]: https://github.com/firecracker-microvm/firecracker/blob/main/docs/snapshotting/handling-page-faults-on-snapshot-resume.md
> [5]: https://lore.kernel.org/kvm/20250618042424.330664-1-jthoughton@google.com
> [6]: https://lore.kernel.org/linux-mm/20250627154655.2085903-1-peterx@redhat.com
> [7]: https://lore.kernel.org/lkml/20240222-gunyah-v17-0-1e9da6763d38@quicinc.com
> [8]: https://git.kernel.org/pub/scm/linux/kernel/git/david/linux.git/log/?h=guestmemfd-preview
> [9]: https://lore.kernel.org/kvm/20250902111951.58315-1-kalyazin@amazon.com
> 
> Nikita Kalyazin (2):
>   mm: guestmem: introduce guestmem library
>   userfaulfd: add minor mode for guestmem
> 
>  Documentation/admin-guide/mm/userfaultfd.rst |   4 +-
>  MAINTAINERS                                  |   2 +
>  fs/userfaultfd.c                             |   3 +-
>  include/linux/guestmem.h                     |  46 +++
>  include/linux/userfaultfd_k.h                |   8 +-
>  include/uapi/linux/userfaultfd.h             |   8 +-
>  mm/Kconfig                                   |   3 +
>  mm/Makefile                                  |   1 +
>  mm/guestmem.c                                | 380 +++++++++++++++++++
>  mm/userfaultfd.c                             |  14 +-
>  virt/kvm/Kconfig                             |   1 +
>  virt/kvm/guest_memfd.c                       | 303 ++-------------
>  12 files changed, 493 insertions(+), 280 deletions(-)
>  create mode 100644 include/linux/guestmem.h
>  create mode 100644 mm/guestmem.c
> 
> 
> base-commit: 911634bac3107b237dcd8fdcb6ac91a22741cbe7
> -- 
> 2.50.1
> 
> 
> 

-- 
Peter Xu