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(UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1696841021; bh=otc7OfNhraJmkEuE8nhHuV118f/GVw4nI6P2etVcZfs=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=KE5jyaI1vXYs5g40TIRVUzC931PNUBTA7Fd/tsfO4yKfdpIWV/3g2bhtQ9OA/dC+3 6nYhS4PcDvnSJnIxxc6OQZVTP1KtsOsBFDTrEL5E56HyzHB2puTyFUB1MNwCAYwM9u Oaf1EZO7dLr1Xl1rPzFrWaTCIFdCSPRjgvvuL1pao+N66Mf1nox1uxmQYqSTrAenOb cPFf1BW0jqctmUZL1KGkRTKUTi04awyxW2HZfAcfqy89skDkXl3N7xAiO6CK4VUmut SuccwWgOfzOLFnsQFvPOuHTIyACKTCOVWUcPi068bjYFRD37o6LdCuYDybW8CQ9GQs gH45+RfmowMQQ== Date: Mon, 9 Oct 2023 11:42:47 +0300 From: Mike Rapoport To: Axel Rasmussen Cc: Alejandro Colomar , 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: <20231009084247.GI3303@kernel.org> References: <20230919190206.388896-1-axelrasmussen@google.com> <20230919190206.388896-6-axelrasmussen@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230919190206.388896-6-axelrasmussen@google.com> X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: B5E7A140004 X-Stat-Signature: i1ngbk4orpqm3sbz6yuo6frtbqb5hrbb X-HE-Tag: 1696841023-951332 X-HE-Meta: U2FsdGVkX1/Ujro08CMZX1oFa/IwlnbEW/BUUED4hBF/b8N+NaDaxabV1knlt1vXTp8jQ6VFaH2sVp6hLWjpTYzOZUgszkJKBcUaAS4BtT/IdWMAlgMC8BmWNM7SAh3YjpbHsAm+jApmx41d/QHBsjwxScyxEwMigsrF2j2mqkGo6wfOpla6BOcPFWOXdY4ERR1evppzx+HJTEjRzqB8MV8aLRh73DCanTrPGngyPPZNv/3gN/6VNwCkFemSKSDjj9t5b7Ov1K/dBxJjkcGOpASi+g7UR/RmI2i5sW7hyQPPyPeRzGM9xQe71qfY+nG1ERIs5uF1heWGz8qVXu/FsYAcQiWnBmvpsB1FWMZzqa+15fWsY6HJ4VC/thoW4YXoVgUqHqYkLGgD4/MYUCK4mzr7vqRv8KrbdTAa+cAI0WDAbzkB6c1NRe5mzDty0BATcpAQBvxJxlW0a9OX2m0S2ZYW95W5RTRL/Umf2bHAE1LY+KnqHIAneGBWzl+JCRgPAh/NsoOqmiQL1TW8qXBRTlzRA7lTcEQWstW/fHS5HEJfrFJefMDyaecEIvfU+XSpbgG6IaApgLYJ6bwO+KwYDQkj5wDqtL7CHOglfyu0DWV664/LiCuyNsGUykLT0sbaVRj5K7o8ziUoIpD5g46HJ0r14KTONLosyB6xWggiz+mIPPYKUKIHurg7aqPCm4GCOTt7JW2ViND9Tl5aNZPPEb5jvdnVobo5goctbBLPdPkgOCfgOtQcrlOEJ6HeU+oLLEbaL2kdWgmcgTUr1eBdjoxmnLaZ3CrfbWujmHIzOvURW/D77ffiU59u58Hv2apR4y2MAluevi48KxKSGJ+tc3BAqPRyjCal9j9/qAlV2ZQ4xIJSuSTpYhTyDWRJ24IweusbYXN+1f/k1wAKBK/UwJNFWDFKnMjloCQAeOKL+5VnxMAaQ4Zbk2UPKN47dgOb2RJlbGA7Yv1dMeX4Rvo LpdWmldk yzdsCZkThjXonR5PIfVB6Uak2a3zk7wee7wZ4tWJpxxWrNkWVc1inHqZeZEBWLl0QEnIoqk08DLuysNALUNZHAd7V/cu2c0f+37kCYhVHERryUUGx2GLfy4yMiku1RXrovl5oYRoyd8FdrmXQdJbtJ3xnE8JBsBoaImmW40A06POZ9Ah8gkBdcrUUMGuLu5x6q3n/9ao0nZtgj3z3Mutz+t9GxCYXZJYm+k4naVYA3d7Ihx4HRtGrVKIiSQ7VFyr+C455tuSRw6mdQ/hDQbmpmDwMKJGKJDMy5wqqGk9kRpQURGS/nlJ75PqXcfHs8UEPd6kw 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: 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). > > This lets us clean up an old FIXME asking for this to be described. > > Signed-off-by: Axel Rasmussen Reviewed-by: Mike Rapoport (IBM) > --- > 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 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 features 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. > +.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 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 > -- > 2.42.0.459.ge4e396fd5e-goog > > -- Sincerely yours, Mike.