[PATCH RFC v2 0/5] mm: Introduce guest_memfd library

[Elliot Berman <quic_eberman@quicinc.com>]

In preparation for adding more features to KVM's guest_memfd, refactor
and introduce a library which abstracts some of the core-mm decisions
about managing folios associated with the file. The goal of the refactor
serves two purposes:

1. Provide an easier way to reason about memory in guest_memfd. With KVM
supporting multiple confidentiality models (TDX, SEV-SNP, pKVM, ARM
CCA), and coming support for allowing kernel and userspace to access
this memory, it seems necessary to create a stronger abstraction between
core-mm concerns and hypervisor concerns.

2. Provide a common implementation for other hypervisors (Gunyah) to use.

To create a guest_memfd, the owner provides operations to attempt to
unmap the folio and check whether a folio is accessible to the host. The
owner can call guest_memfd_make_inaccessible() to ensure Linux doesn't
have the folio mapped.

The series first introduces a guest_memfd library based on the current
KVM (next) implementation, then adds few features needed for Gunyah and
arm64 pKVM. The Gunyah usage of the series will be posted separately
shortly after sending this series. I'll work with Fuad on using the
guest_memfd library for arm64 pKVM based on the feedback received.

There are a few TODOs still pending. 
- The KVM patch isn't tested. I don't have access a SEV-SNP setup to be
  able to test.
- I've not yet investigated deeply whether having the guest_memfd
  library helps live migration. I'd appreciate any input on that part.
- We should consider consolidating the adjust_direct_map() in
  arch/x86/virt/svm/sev.c so guest_memfd can take care of it.
- There's a race possibility where the folio ref count is incremented
  and about to also increment the safe counter, but waiting for the
  folio lock to be released. The owner of folio_lock will see mismatched
  counter values and not be able to convert to (in)accessible, even
  though it should be okay to do so.
 
I'd appreciate any feedback, especially on the direction I'm taking for
tracking the (in)accessible state.

Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>

Changes in v2:
- Significantly reworked to introduce "accessible" and "safe" reference
  counters
- Link to v1:
  https://lore.kernel.org/r/20240805-guest-memfd-lib-v1-0-e5a29a4ff5d7@quicinc.com

---
Elliot Berman (5):
      mm: Introduce guest_memfd
      mm: guest_memfd: Allow folios to be accessible to host
      kvm: Convert to use guest_memfd library
      mm: guest_memfd: Add ability for userspace to mmap pages
      mm: guest_memfd: Add option to remove inaccessible memory from direct map

 arch/x86/kvm/svm/sev.c      |   3 +-
 include/linux/guest_memfd.h |  49 ++++
 mm/Kconfig                  |   3 +
 mm/Makefile                 |   1 +
 mm/guest_memfd.c            | 667 ++++++++++++++++++++++++++++++++++++++++++++
 virt/kvm/Kconfig            |   1 +
 virt/kvm/guest_memfd.c      | 371 +++++-------------------
 virt/kvm/kvm_main.c         |   2 -
 virt/kvm/kvm_mm.h           |   6 -
 9 files changed, 797 insertions(+), 306 deletions(-)
---
base-commit: 5be63fc19fcaa4c236b307420483578a56986a37
change-id: 20240722-guest-memfd-lib-455f24115d46

Best regards,
-- 
Elliot Berman <quic_eberman@quicinc.com>