From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932376Ab1EQVcW (ORCPT ); Tue, 17 May 2011 17:32:22 -0400 Received: from mga14.intel.com ([143.182.124.37]:3648 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932269Ab1EQVcV (ORCPT ); Tue, 17 May 2011 17:32:21 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.65,227,1304319600"; d="scan'208";a="437835286" From: Andi Kleen To: Andy Lutomirski Cc: Ingo Molnar , Thomas Gleixner , x86@kernel.org, linux-kernel@vger.kernel.org, Linus Torvalds , "David S. Miller" , Eric Dumazet , Peter Zijlstra , Borislav Petkov Subject: Re: [PATCH v4 0/6] Micro-optimize vclock_gettime References: <20110516160943.GC25898@one.firstfloor.org> <20110516164939.GD25898@one.firstfloor.org> <20110517080029.GB22093@elte.hu> <20110517113634.GC13475@elte.hu> <4DD2BEFB.6070609@mit.edu> Date: Tue, 17 May 2011 14:31:26 -0700 In-Reply-To: <4DD2BEFB.6070609@mit.edu> (Andy Lutomirski's message of "Tue, 17 May 2011 14:31:23 -0400") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/23.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Andy Lutomirski writes: > > On KVM on Sandy Bridge, I can emulate a vsyscall that does nothing in 400ns or so. I'll try to make this code emulate real vsyscalls over the weekend. This was much easier than I expected. How about the performance of all the statically linked programs? I guess you just declared they don't matter? gettimeofday is quite critical and adding a exception into it is just a performance desaster. Also it's always a dangerous assumption to think that all programs on Linux use glibc ("all world is a Vax") In fact more and more of Linux users are using different libcs these days (like Android or embedded systems or languages with special runtime systems) Who knows if all those other libraries use vDSO? And then there are of course the old glibcs. A lot of people (including me) use new kernels with old userland. For me this seems like a very risky move -- breaking performance of previously perfectly good set ups for very little reason. Given the old vsyscall code is somewhat ugly -- I wouldn't argue that -- but compatibility (including performance compatibility) has always been importand in Linux and we have far uglier code around in the name of compatibility.. And the "security problem" you keep talking about can be fixed much easier and more compatible as I pointed out. As far as I'm concered the change is a bad idea. -Andi -- ak@linux.intel.com -- Speaking for myself only