[PATCH 03/10] userfaultfd.2: comment on feature detection in the example program

[Axel Rasmussen <axelrasmussen@google.com>]

The example program doesn't depend on any extra features, so it does not
make use of the two-step feature handshake process. This is fine, but it
might set a bad example for programs which *do* depend on specific
features (e.g. they may conclude they don't need to do anything to
enable / detect them).

No need to make the example program more complicated: let's just add a
comment indicating why we do it the way we do it in the example, and
describing briefly what a more complicated program would need to do
instead.

The comment is kept rather brief; a full description of this feature
will be included in ioctl_userfaultfd.2 instead.

Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 man2/userfaultfd.2 | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
index 00d94e514..b2b79f61d 100644
--- a/man2/userfaultfd.2
+++ b/man2/userfaultfd.2
@@ -881,6 +881,13 @@ main(int argc, char *argv[])
     if (uffd == \-1)
         err(EXIT_FAILURE, "userfaultfd");
 \&
+    /* NOTE: Two-step feature handshake is not needed here, since this
+       example doesn't require any specific features.
+
+       Programs that *do* should call UFFDIO_API twice: once with
+       `features = 0` to detect features supported by this kernel, and
+       again with the subset of features the program actually wants to
+       enable. */
     uffdio_api.api = UFFD_API;
     uffdio_api.features = 0;
     if (ioctl(uffd, UFFDIO_API, &uffdio_api) == \-1)
-- 
2.42.0.459.ge4e396fd5e-goog