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b=DZye6kFSL5tEFdCjs6MTkk47EvSnsY2Nre6JIrO0oUkmZGhBkTxD4fw/6M2ik+6UH OBaW2AcdSVzxvRAfU1YsDqLBjItnHo70Xgcre3gbmhvUWUzhYCbuvppcC8n6ovUbbA d2gaYsklkkBN+k3GvWEybOTdQ5C1+tnV5e94K8VhIoaemsBCW2K8ZjCL2ZXvcf16s1 5SaqClDch5QdecmGk5sGuuGEFSyBsRwtoOfrh0YVFUehMqY0IeJyh+z38Y0DD48uJL 38td8l2QsNxmDuvBATmuYgSS5bspoLX52/Z+cHkY3hZwtfyLyK+leO2tkdvVDxhbvJ 4GwmYAnYaJZTw== Date: Mon, 9 Oct 2023 12:58:21 +0200 From: Alejandro Colomar To: Mike Rapoport Cc: Axel Rasmussen , Peter Xu , linux-man@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 05/10] ioctl_userfaultfd.2: describe two-step feature handshake Message-ID: References: <20230919190206.388896-1-axelrasmussen@google.com> <20230919190206.388896-6-axelrasmussen@google.com> <20231009084247.GI3303@kernel.org> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha512; protocol="application/pgp-signature"; boundary="W7HwIlpxFh93S0s/" Content-Disposition: inline In-Reply-To: <20231009084247.GI3303@kernel.org> X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 825B4140011 X-Stat-Signature: 36u8uhgxp91zpathgyso5hzufpwuanxp X-Rspam-User: X-HE-Tag: 1696849107-770290 X-HE-Meta: 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 wlqGyq19 G98iI1PjJgR7a0LZ65HmDamAbWJJT1N24a8VrU1O1v04ZIN76OChlk7Z7J9l5g8RJy+TJQ19Rf1FZVQeZNCCfcVx+QMJYszIwr6+oqR2A/IGFOj8FMh4hQTQsQpVhij2KdfaD9FmkslTNMqESJr7ghT814P0m4fZ67KQ7ZyS4mkolMg5pioyhQarHGHEhEAiVWj23dsXI8itcuMmM0xzMdoWt9jdwJBpHiAmN6vgNkT2ThtEaCWDY9spNwN4RCxnqLOUseN9Gp/w/9UFXsQMqmwguutRffvyK6e/0oNr6WqlN8yqvIXEtX12SNyOSm3QahJZM4qkbkgFIUXyMLJZGP8Xt0lXoyo9GhBnXJnYc9rxovl4rmqB8AW4s+4vCKaL6xw17ufSMn5G0+wf8gyKTpuAbzFIQdlTMaCiVoOlOQMBPgO6OyhYYDKgM/NqtlsHdLUZc4wmBZ/2R+A6fGe3Fp+BjFrQ5/26RJ/FW 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: --W7HwIlpxFh93S0s/ Content-Type: text/plain; protected-headers=v1; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable Date: Mon, 9 Oct 2023 12:58:21 +0200 From: Alejandro Colomar To: Mike Rapoport Cc: Axel Rasmussen , Peter Xu , linux-man@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 05/10] ioctl_userfaultfd.2: describe two-step feature handshake 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). > >=20 > > This lets us clean up an old FIXME asking for this to be described. > >=20 > > Signed-off-by: Axel Rasmussen >=20 > Reviewed-by: Mike Rapoport (IBM) Since v2 is unchanged, I've added this tag. Thanks for the review! Cheers, Alex >=20 > > --- > > 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 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 f= eatures 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 defaul= t. > > -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 > > 2.42.0.459.ge4e396fd5e-goog > >=20 > >=20 >=20 > --=20 > Sincerely yours, > Mike. --=20 --W7HwIlpxFh93S0s/ Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAABCgAdFiEE6jqH8KTroDDkXfJAnowa+77/2zIFAmUj3M0ACgkQnowa+77/ 2zIljg/+O+XKXPmc+OxbhyCJMh2jHqOEpdFbUq6rqZM6ZkTGGSD+G4lM4tN7QNvd 9UYN/jrxFq/Og4zpCYULSxSqYwgnKC25e3l2KNK0wT49D/T05vl/Fq2zQSD7lL2/ 303w2aAKijCdicWZumFytoUxAVybgHvsDGqgyq65w57muAWV+mSQjIQYnpXIvBKw uA1wncaMpXTU0CG7uehIGbudIRrc1FpmNp1yPjSMyCeN8LucS8xTOwDOaf1a1Q4L X4pgP/sr8t2uKC/1M9rnaA8GvhZp9vwxHBJxBvKyVhmkoiD8Fc15AtjI+OBKuo60 6Wh2jsrKQZsSeuYU/pITUO2XszbckymBatAIQkAbMPsQODthglIfb59GGCTxLoVU MJ88Pguc/WaSFz06T+AErgJaT94dnup4o2oDSloeQ+etRR0Ooqf5Dsm2cImRoIMt wKwYcJuYivqex+y85tBiD3UMIrKzpyyb3oTaYEfHMWpAzQnUYS7HJ0TAEW+pLF+f g8G8SohJkpxpkZ4SQKfs719tCB7qkUR0ToQhi18q2vGaQMjbFU48xZKidyV8bBju HwU2DpsYG6Iw0F7K1dcF3lACdvMhcdbOLygcM+tSDGD2asntQ6qEMn4EjgX18fCg kOLjOIYUJRaGMWxRBetts4DI41BN394UznxLu93XjJ+WVdSwV3k= =BdwT -----END PGP SIGNATURE----- --W7HwIlpxFh93S0s/--