Re: [PATCH 05/10] ioctl_userfaultfd.2: describe two-step feature handshake

Mike Rapoport
2023-10-09T08:43:44Z

On Tue, Sep 19, 2023 at 12:02:01PM -0700, Axel Rasmussen wrote:
> Fully describe how UFFDIO_API can be used to perform a two-step feature
> handshake, and also note the case where this isn't necessary (programs
> which don't depend on any extra features).
>
> This lets us clean up an old FIXME asking for this to be described.
>
> Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>

Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>

> ---
> man2/ioctl_userfaultfd.2 | 37 +++++++++++++++++++++----------------
> 1 file changed, 21 insertions(+), 16 deletions(-)
>
> diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
> index 339adf8fe..e91a1dfc8 100644
> --- a/man2/ioctl_userfaultfd.2
> +++ b/man2/ioctl_userfaultfd.2
> @@ -83,7 +83,6 @@ struct uffdio_api {
> The
> .I api
> field denotes the API version requested by the application.
> -.PP
> The kernel verifies that it can support the requested API version,
> and sets the
> .I features
> @@ -93,6 +92,25 @@ fields to bit masks representing all the available features and the generic
> .BR ioctl (2)
> operations available.
> .PP
> +After Linux 4.11,
> +applications should use the
> +.I features
> +field to perform a two-step handshake.
> +First,
> +.BR UFFDIO_API
> +is called with the
> +.I features
> +field set to zero.
> +The kernel responsds by setting all supported feature bits.
> +.PP
> +Applications which do not require any specific features
> +can begin using the userfaultfd immediately.
> +Applications which do need specific features
> +should call
> +.BR UFFDIO_API
> +again with a subset of the reported feature bits set
> +to enable those features.
> +.PP
> Before Linux 4.11, the
> .I features
> field must be initialized to zero before the call to
> @@ -102,24 +120,11 @@ and zero (i.e., no feature bits) is placed in the
> field by the kernel upon return from
> .BR ioctl (2).
> .PP
> -Starting from Linux 4.11, the
> -.I features
> -field can be used to ask whether particular features are supported
> -and explicitly enable userfaultfd features that are disabled by default.
> -The kernel always reports all the available features in the
> -.I features
> -field.
> -.PP
> -To enable userfaultfd features the application should set
> -a bit corresponding to each feature it wants to enable in the
> -.I features
> -field.
> -If the kernel supports all the requested features it will enable them.
> -Otherwise it will zero out the returned
> +If the application sets unsupported feature bits,
> +the kernel will zero out the returned
> .I uffdio_api
> structure and return
> .BR EINVAL .
> -.\" FIXME add more details about feature negotiation and enablement
> .PP
> The following feature bits may be set:
> .TP
> --
> 2.42.0.459.ge4e396fd5e-goog
>
>

--
Sincerely yours,
Mike.