[PATCH v7 0/2] KVM: guest_memfd: use write for population

["Kalyazin, Nikita" <kalyazin@amazon.co.uk>]

On systems that support shared guest memory, write() is useful, for
example, for population of the initial image.  Even though the same can
also be achieved via userspace mapping and memcpying from userspace,
write() provides a more performant option because it does not need to
set user page tables and it does not cause a page fault for every page
like memcpy would.  Note that memcpy cannot be accelerated via
MADV_POPULATE_WRITE as it is not supported by guest_memfd and relies on
GUP.

Populating 512MiB of guest_memfd on a x86 machine:
 - via memcpy: 436 ms
 - via write:  202 ms (-54%)

Only PAGE_ALIGNED offset and len are allowed.  Even though non-aligned
writes are technically possible, when in-place conversion support is
implemented [1], the restriction makes handling of mixed shared/private
huge pages simpler.  write() will only be allowed to populate shared
pages.

When direct map removal is implemented [2]
 - write() will not be allowed to access pages that have already
   been removed from direct map
 - on completion, write() will remove the populated pages from
   direct map

While it is technically possible to implement read() syscall on systems
with shared guest memory, it is not supported as there is currently no
use case for it.

[1]
https://lore.kernel.org/kvm/cover.1760731772.git.ackerleytng@google.com
[2]
https://lore.kernel.org/kvm/20250924151101.2225820-1-patrick.roy@campus.lmu.de

Nikita Kalyazin (2):
  KVM: guest_memfd: add generic population via write
  KVM: selftests: update guest_memfd write tests

 Documentation/virt/kvm/api.rst                |  2 +
 include/linux/kvm_host.h                      |  2 +-
 include/uapi/linux/kvm.h                      |  1 +
 .../testing/selftests/kvm/guest_memfd_test.c  | 58 +++++++++++++++++--
 virt/kvm/guest_memfd.c                        | 52 +++++++++++++++++
 5 files changed, 108 insertions(+), 7 deletions(-)


base-commit: 8a4821412cf2c1429fffa07c012dd150f2edf78c
--
2.50.1