[PATCH RFC 0/4] 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 sepately
shortly after sending this series. I'll work with Fuad on using the
guest_memfd library for arm64 pKVM based on the feedback received.

I've not yet investigated deeply whether having the guest_memfd library
helps live migration. I'd appreciate any input on that part.

Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
---
Elliot Berman (4):
      mm: Introduce guest_memfd
      kvm: Convert to use mm/guest_memfd
      mm: guest_memfd: Add option to remove guest private memory from direct map
      mm: guest_memfd: Add ability for mmap'ing pages

 include/linux/guest_memfd.h |  59 ++++++
 mm/Kconfig                  |   3 +
 mm/Makefile                 |   1 +
 mm/guest_memfd.c            | 427 ++++++++++++++++++++++++++++++++++++++++++++
 virt/kvm/Kconfig            |   1 +
 virt/kvm/guest_memfd.c      | 299 +++++--------------------------
 virt/kvm/kvm_main.c         |   2 -
 virt/kvm/kvm_mm.h           |   6 -
 8 files changed, 539 insertions(+), 259 deletions(-)
---
base-commit: 8400291e289ee6b2bf9779ff1c83a291501f017b
change-id: 20240722-guest-memfd-lib-455f24115d46

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