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 X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 16117C3A589 for ; Thu, 15 Aug 2019 18:03:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id EC7CB2083B for ; Thu, 15 Aug 2019 18:03:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731882AbfHOSD2 (ORCPT ); Thu, 15 Aug 2019 14:03:28 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:40508 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728500AbfHOSD2 (ORCPT ); Thu, 15 Aug 2019 14:03:28 -0400 Received: from pd9ef1cb8.dip0.t-ipconnect.de ([217.239.28.184] helo=nanos) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1hyK5X-0002zv-Le; Thu, 15 Aug 2019 20:03:03 +0200 Date: Thu, 15 Aug 2019 20:03:02 +0200 (CEST) From: Thomas Gleixner To: Dmitry Safonov cc: linux-kernel@vger.kernel.org, Dmitry Safonov <0x7f454c46@gmail.com>, Andrei Vagin , Adrian Reber , Andrei Vagin , Andy Lutomirski , Arnd Bergmann , Christian Brauner , Cyrill Gorcunov , "Eric W. Biederman" , "H. Peter Anvin" , Ingo Molnar , Jann Horn , Jeff Dike , Oleg Nesterov , Pavel Emelyanov , Shuah Khan , Vincenzo Frascino , containers@lists.linux-foundation.org, criu@openvz.org, linux-api@vger.kernel.org, x86@kernel.org Subject: Re: [PATCHv6 25/36] vdso: Introduce vdso_static_branch_unlikely() In-Reply-To: <20190815163836.2927-26-dima@arista.com> Message-ID: References: <20190815163836.2927-1-dima@arista.com> <20190815163836.2927-26-dima@arista.com> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 15 Aug 2019, Dmitry Safonov wrote: > From: Andrei Vagin > > As it has been discussed on timens RFC, adding a new conditional branch > `if (inside_time_ns)` on VDSO for all processes is undesirable. > > Addressing those problems, there are two versions of VDSO's .so: > for host tasks (without any penalty) and for processes inside of time > namespace with clk_to_ns() that subtracts offsets from host's time. > > Introduce vdso_static_branch_unlikely(), which is similar to > static_branch_unlikely(); alias it with timens_static_branch_unlikely() > under CONFIG_TIME_NS. > > The timens code in vdso will look like this: > > if (timens_static_branch_unlikely()) { > clk_to_ns(clk, ts); Please name that clk_to_namespace(). _ns() is widely used for nanoseconds. > } > > The version of vdso which is compiled from sources will never execute > clk_to_ns(). And then we can patch the 'no-op' in the straight-line > codepath with a 'jump' instruction to the out-of-line true branch and > get the timens version of the vdso library. Colour me confused. Why do we need that static branch at all? Why don't we compile VDSO_NO_NAMESPACE and VDSO_NAMESPACE right away? One has the clk_to_namespace() one does not. The you can spare the whole static key patching and the NO_NAMESPACE variant does not have extra 5 NOPS. The VDSO is one page IIRC, so having the extra namespace variant around does really not matter at all. Thanks, tglx