Re: [PATCH v2 1/5] ioctl_userfaultfd.2: describe two-step feature handshake

[Alejandro Colomar]

On Tue, Oct 03, 2023 at 12:45:43PM -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@xxxxxxxxxx>

Patch applied.

Thanks,
Alex

> ---
>  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 b5281ec4c..ef352a69d 100644
> --- a/man2/ioctl_userfaultfd.2
> +++ b/man2/ioctl_userfaultfd.2
> @@ -82,7 +82,6 @@ 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
> @@ -92,6 +91,25 @@ fields to bit masks representing all the available features and the generic
>  .BR ioctl (2)
>  operations available.
>  .PP
> +Since 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
> @@ -101,24 +119,11 @@ 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
> -- 
> 2.42.0.609.gbb76f46606-goog
> 

-- 
<https://www.alejandro-colomar.es/>

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