[ based on kvm/next ] Implement guest_memfd population via the write syscall. This is useful in non-CoCo use cases where the host can access guest memory. 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 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%) The write syscall support is conditional on kvm_gmem_supports_mmap. When in-place shared/private conversion is supported, write should only be allowed on shared pages. v6: - Make write support conditional on mmap support instead of relying on the up-to-date flag to decide whether writing to a page is allowed - James: Remove depenendencies on folio_test_large - James: Remove page alignment restriction - James: Formatting fixes v5: - https://lore.kernel.org/kvm/[email protected]/ - Replace the call to the unexported filemap_remove_folio with zeroing the bytes that could not be copied - Fix checkpatch findings v4: - https://lore.kernel.org/kvm/[email protected] - Switch from implementing the write callback to write_iter - Remove conditional compilation v3: - https://lore.kernel.org/kvm/[email protected] - David/Mike D: Only compile support for the write syscall if CONFIG_KVM_GMEM_SHARED_MEM (now gone) is enabled. v2: - https://lore.kernel.org/kvm/[email protected] - Switch from an ioctl to the write syscall to implement population v1: - https://lore.kernel.org/kvm/[email protected] Nikita Kalyazin (2): KVM: guest_memfd: add generic population via write KVM: selftests: update guest_memfd write tests .../testing/selftests/kvm/guest_memfd_test.c | 51 ++++++++++++++++--- virt/kvm/guest_memfd.c | 49 ++++++++++++++++++ 2 files changed, 94 insertions(+), 6 deletions(-) base-commit: 6b36119b94d0b2bb8cea9d512017efafd461d6ac -- 2.50.1
