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Date:   Tue,  3 Oct 2023 12:45:43 -0700
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Subject: [PATCH v2 1/5] ioctl_userfaultfd.2: describe two-step feature handshake
From:   Axel Rasmussen <axelrasmussen@google.com>
To:     Alejandro Colomar <alx@kernel.org>, Peter Xu <peterx@redhat.com>
Cc:     linux-man@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Axel Rasmussen <axelrasmussen@google.com>
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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>
---
 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