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=-0.8 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 B6FFEC43143 for ; Thu, 13 Sep 2018 19:07:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 63D4F20866 for ; Thu, 13 Sep 2018 19:07:42 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 63D4F20866 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728258AbeINAS2 (ORCPT ); Thu, 13 Sep 2018 20:18:28 -0400 Received: from mx1.redhat.com ([209.132.183.28]:34298 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727277AbeINAS2 (ORCPT ); Thu, 13 Sep 2018 20:18:28 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id E905D87625; Thu, 13 Sep 2018 19:07:39 +0000 (UTC) Received: from oldenburg.str.redhat.com (ovpn-116-49.ams2.redhat.com [10.36.116.49]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 19FF95D6AA; Thu, 13 Sep 2018 19:07:37 +0000 (UTC) Subject: Re: [PATCH v3] x86/vdso: Handle clock_gettime(CLOCK_TAI) in vDSO To: Andy Lutomirski Cc: Thomas Gleixner , Andy Lutomirski , Matt Rickard , Stephen Boyd , John Stultz , X86 ML , LKML References: <20180901015935.CCF0B18E20A9@virtux64.softrans.com.au> <59403142-1073-3926-65dd-ac55ead357b9@redhat.com> <0120ea99-fcac-839a-ef8f-1daa74c17bec@redhat.com> From: Florian Weimer Message-ID: <5f9e890c-fd67-2e39-42ca-64290301b20a@redhat.com> Date: Thu, 13 Sep 2018 21:07:36 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.26]); Thu, 13 Sep 2018 19:07:40 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/13/2018 05:22 PM, Andy Lutomirski wrote: > > >> On Sep 13, 2018, at 1:07 AM, Florian Weimer wrote: >> >> On 09/12/2018 07:11 PM, Andy Lutomirski wrote: >>>> The multiplexer interfaces need much more surgery and talking about futex, >>>> we'd need to sit down with quite some people and identify the things they >>>> actually care about before just splitting it up and keeping the existing >>>> overloaded trainwreck the same. >>>> >>> There’s also the issue of how much the speedup matters. For futex, maybe a better interface saves 3ns, but a futex syscall is hundreds of ns. clock_gettime() is called at high frequency and can be ~25ns. Saving a few ns is a bigger deal. >> >> My concern is that the userspace system call wrappers currently do not know how many arguments the individual operations take and what types the arguments have (hence the “type-polymorphic” nature I mentioned). This could be a problem for on-stack argument passing (where you might read values beyond the end of the stack, and glibc avoids that most of the time by having enough cruft on the stack), and for architectures which pass pointers and integers in different registers (like some m68k ABIs do for the return value). > Isn’t clock_gettime already special because of the vDSO entry point, though? Somewhat special, yes, but not overly so, and not in the type-polymorphic sense. We can't give direct access of the vDSO implementation to applications because the kernel does not know about the userspace errno variable. We do that for time on x86_64, where applications call into the vDSO directly, bypassing glibc completely after binding. I suspect most Linux libcs that know about the vDSO at all have generic vsyscall support, just like they have generic support for plain system calls. Thanks, Florian