ioctl_userfaultfd.2: Update UFFDIO_API description

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/ioctl_userfaultfd.2

@@ -95,11 +95,6 @@ struct uffdio_api {
 The
 .I api
 field denotes the API version requested by the application.
-Before the call, the
-.I features
-field must be initialized to zero.
-In the future, it is intended that this field can be used to ask
-whether particular features are supported.
 
 The kernel verifies that it can support the requested API version,
 and sets the
@@ -109,9 +104,40 @@ and
 fields to bit masks representing all the available features and the generic
 .BR ioctl (2)
 operations available.
-Currently, zero (i.e., no feature bits) is placed in the
+
+For Linux kernel versions before 4.11, the
+.I features
+field must be initialized to zero before the call to
+.I UFFDIO_API
+, and zero (i.e., no feature bits) is placed in the
+.I features
+field by the kernel upon return from
+.BR ioctl (2).
+
+Starting from Linux 4.11, the
+.I features
+field can be used to 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.
+.\" FIXME add more details about feature negotiation and enablement
+
+Since Linux 4.11, the following feature bits may be set:
+.TP
+.B UFFD_FEATURE_EVENT_FORK
+.TP
+.B UFFD_FEATURE_EVENT_REMAP
+.TP
+.B UFFD_FEATURE_EVENT_REMOVE
+.TP
+.B UFFD_FEATURE_EVENT_UNMAP
+.TP
+.B UFFD_FEATURE_MISSING_HUGETLBFS
+.TP
+.B UFFD_FEATURE_MISSING_SHMEM
+.\" FIXME add feature description
+
 The returned
 .I ioctls
 field can contain the following bits: