Re: [PATCH v5 09/17] KVM: Introduce KVM_CAP_USERFAULT_ON_MISSING without implementation

[David Matlack <dmatlack@google.com>]

On Fri, Sep 8, 2023 at 3:30 PM Anish Moorthy <amoorthy@google.com> wrote:
>
> Add documentation, memslot flags, useful helper functions, and the
> definition of the capability. Implementation is provided in a subsequent
> commit.
>
> Memory fault exits on absent mappings are particularly useful for
> userfaultfd-based postcopy live migration, where contention within uffd
> can lead to slowness When many vCPUs fault on a single uffd/vma.
> Bypassing the uffd entirely by returning information directly to the
> vCPU via an exit avoids contention and can greatly improves the fault
> rate.
>
> Suggested-by: James Houghton <jthoughton@google.com>
> Signed-off-by: Anish Moorthy <amoorthy@google.com>
> ---
>  Documentation/virt/kvm/api.rst | 28 +++++++++++++++++++++++++---
>  include/linux/kvm_host.h       |  9 +++++++++
>  include/uapi/linux/kvm.h       |  2 ++
>  tools/include/uapi/linux/kvm.h |  1 +
>  virt/kvm/Kconfig               |  3 +++
>  virt/kvm/kvm_main.c            |  5 +++++
>  6 files changed, 45 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 92fd3faa6bab..c2eaacb6dc63 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -1312,6 +1312,7 @@ yet and must be cleared on entry.
>    /* for kvm_userspace_memory_region::flags */
>    #define KVM_MEM_LOG_DIRTY_PAGES      (1UL << 0)
>    #define KVM_MEM_READONLY     (1UL << 1)
> +  #define KVM_MEM_USERFAULT_ON_MISSING  (1UL << 2)

Bike Shedding! Maybe KVM_MEM_EXIT_ON_MISSING? "Exiting" has concrete
meaning in the KVM UAPI whereas "userfault" doesn't and could suggest
going through userfaultfd, which is the opposite of what this
capability is doing.