From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 82342E95A96 for ; Mon, 9 Oct 2023 08:43:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1345474AbjJIInp (ORCPT ); Mon, 9 Oct 2023 04:43:45 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53570 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234407AbjJIInm (ORCPT ); Mon, 9 Oct 2023 04:43:42 -0400 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EDA2397; Mon, 9 Oct 2023 01:43:41 -0700 (PDT) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 930E7C433C7; Mon, 9 Oct 2023 08:43:39 +0000 (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> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.