[PATCH kvmtool v2 0/4] Add guest_memfd support for arm64

[Fuad Tabba <fuad.tabba@linux.dev>]

Hi folks,

Changes since v1 [1]:
- Use kvm_userspace_memory_region2 only for guest_memfd slots (Suzuki)
- Check KVM_CAP_GUEST_MEMFD before KVM_CAP_GUEST_MEMFD_FLAGS (Alexandru)
- Close guest_memfd fd after unmapping at teardown (Alexandru)
- Carried Reviewed-bys

Due to popular demand, i.e., Alexandru asking me about it, I repolished my
guest_memfd support series. The one I sent years ago was stale and
over-engineered, so this is a fresh start instead.

This series adds support for backing guest RAM with guest_memfd on
arm64, for non-protected VMs under both regular KVM and pKVM.

The first two patches are standalone fixes in the code this series
builds on: an uninitialized return value in kvm__for_each_mem_bank(),
and doubled newlines in die() messages. Patch 3 adds guest_memfd
plumbing to kvm__register_mem(), using kvm_userspace_memory_region2
only for guest_memfd-backed slots.

A new --guest-memfd option creates guest RAM from a guest_memfd with
mmap support (GUEST_MEMFD_FLAG_MMAP and GUEST_MEMFD_FLAG_INIT_SHARED,
Linux 6.18+), maps it to provide the userspace mapping, and registers
it with KVM_SET_USER_MEMORY_REGION2.

Protected VMs need in-place shared/private conversion of guest_memfd,
which is still in progress in the kernel [2], so --protected rejects
the option.

Based on kvmtool master (ca0ddafa941d).

Cheers,
/fuad

[1] https://lore.kernel.org/all/20260706083555.302972-1-fuad.tabba@linux.dev/
[2] https://lore.kernel.org/all/20260618-gmem-inplace-conversion-v8-0-9d2959357853@google.com/

Fuad Tabba (4):
  Initialize the return value in kvm__for_each_mem_bank()
  Remove newline from end of die() aborts
  Add support for registering guest_memfd-backed memory regions
  arm64: Add --guest-memfd option to back guest RAM with guest_memfd

 arm64/include/kvm/kvm-config-arch.h |  5 ++-
 arm64/kvm.c                         | 43 +++++++++++++++++++++-----
 hw/cfi_flash.c                      |  2 +-
 include/kvm/kvm.h                   | 16 ++++++++--
 include/kvm/util.h                  |  1 +
 kvm.c                               | 42 ++++++++++++++++++--------
 util/util.c                         | 47 ++++++++++++++++++++++++++---
 7 files changed, 127 insertions(+), 29 deletions(-)

-- 
2.39.5