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b=YKXgdQRzxz98aBdg4Lu6qe11VhTNIeyrgVUmZMIZ2tm8BujGs4Pwd3iI8w5KO9dur U/kHZbKiae+9ZE8wKBaAtWHmP/7swn20fYA0ByglqLQ3RJE+DpxozNNxVa/GuXt0n3 qWg99YOGA6LJKEKNmktIL27OIuLXqyslNf4E3WhctIMgBNGJ4q8Xkgizny5HiG9dnU YcdcXrgXLpwmZaBDqpgKSXC6hFEF7j69svBwQtioD4BYondUv9QjBi9Z8Pw9WcDmRA lsVV3lY2ESMlJIVfbUCXDco+v+xgEdq+KDHrbxKZWfpoQeQpf47++uSCg4Fg7F8lIg wqfgWGztaCCNQ== Date: Mon, 9 Oct 2023 00:23:49 +0200 From: Alejandro Colomar To: Axel Rasmussen Cc: Peter Xu , linux-man@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/5] ioctl_userfaultfd.2: describe two-step feature handshake Message-ID: References: <20231003194547.2237424-1-axelrasmussen@google.com> <20231003194547.2237424-2-axelrasmussen@google.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="IEa6GDqAgBshCuXi" Content-Disposition: inline In-Reply-To: <20231003194547.2237424-2-axelrasmussen@google.com> X-Rspamd-Queue-Id: A9AED40003 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: hof6qo9riy81o3hxujfuangco6ok4wsp X-HE-Tag: 1696803835-628828 X-HE-Meta: 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 jH8daPaP 4tDidWgXDZGyUYKybPD3l9QQms7A5+Q/sNH59IBfSTo7RUCZ5hhNwruqd9zpNJwLTG9XBkTKocNzFIbjECbVIQ0LTa5hD06au0OpAeiXc2nm9TCoEtERmBhPExnzJqC+drhec2/3dPbAqaR5yos639ezEwlXgBqgTi8cpFazJ3PR5fn72fKm5meIMOGNWYUnPeo4CUvGmGOY2RofHbAD1yTJUlAznHhrdWgHc6KEPk9qSbRX8irU0Hikg1+vecmd29QjMlggNLNKI2G9vjJnDXkCWkN2VCiNSS9lu6RRfjdx+X945TfieHd6/3Z7Ex2JS6gyaaS3mRgB9hwItTOJKt7srap2tDc1AYQVXXXN3hbCBMmSa4goGI5It5+6d0GhXS/UoDbiWnjTvRksWnmO/bvkoMh1PdvELsiKdmBLMqW4i/iA= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: --IEa6GDqAgBshCuXi Content-Type: text/plain; protected-headers=v1; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Date: Mon, 9 Oct 2023 00:23:49 +0200 From: Alejandro Colomar To: Axel Rasmussen Cc: Peter Xu , linux-man@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/5] ioctl_userfaultfd.2: describe two-step feature handshake 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). >=20 > This lets us clean up an old FIXME asking for this to be described. >=20 > Signed-off-by: Axel Rasmussen Patch applied. Thanks, Alex > --- > man2/ioctl_userfaultfd.2 | 37 +++++++++++++++++++++---------------- > 1 file changed, 21 insertions(+), 16 deletions(-) >=20 > 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 fea= tures 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 > --=20 > 2.42.0.609.gbb76f46606-goog >=20 --=20 --IEa6GDqAgBshCuXi Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAABCgAdFiEE6jqH8KTroDDkXfJAnowa+77/2zIFAmUjK/UACgkQnowa+77/ 2zLptg/+P9U2SpowseEvKRg0lV7o/eyhXKRhaPF4TSMq3GLRcTCcSGj/VGCew/nO bz/9ClFGKbXgPf1eyRMwds14/3NhCMf6I6GlCvbhs2dKgXf2xiMYaCZ0L0AtDg/W a9VK6k6BO58V/WEbIQgf3ckY7wJpLpIlcF9FQDSe/5az4DARKxEqhdKoP0OvAIbQ bkBl/BGwaOlJTc/ByTzxJvFZYyUUjp1t0Ae92ydSlFdYSe61Q1QdvxVbVV7c9sUO WXW0Osn3eVe5XjlAttZ60Q+iQO3bnGNOX8gLTZlsTBp52stvmpt+YipmkVqTSE0Q wXNjcgqfgWtkZbvpuWTiQ3YnhWn0WPaQ0EO9KQacIaQnRS654AK6tQZoBUgwO+Nh jiZjvroZ8Fj5QYvzB5KSlsVRL1HHITd0mIc1xA1elks693jDwkH2QjJjUZS+EkZx 9p9ZGbYdGxrZiM5fUoML2GsAEtFDQODorER8FzzzaOoobjsWCQudfUJ9BfmhDrPC Do5cHT8J9ARZJes+Z4yBF8xqPaXRP++QLs7jln3vhr3R5yvQSGVRGb3SoyIiO9Js uDXOdS5GrQhVtXECJVGTL0GXk/NPvj7YAa5/jeU0oYSm6A2b0tnLhY9z4slexLbV mTqiDNna3dTumwPAuGYVY6d2x5QdN4Wql2jhn4roOaONH5W4qPw= =bV+4 -----END PGP SIGNATURE----- --IEa6GDqAgBshCuXi--