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

Alejandro Colomar
2023-10-09T10:58:29Z

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Hi Mike,

On Mon, Oct 09, 2023 at 11:42:47AM +0300, Mike Rapoport wrote:
> 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>

Since v2 is unchanged, I've added this tag. Thanks for the review!

Cheers,
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 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.

s/responsds/responds/ amended.

> > +.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.

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

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