From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753801AbaIXOxQ (ORCPT ); Wed, 24 Sep 2014 10:53:16 -0400 Received: from gw-1.arm.linux.org.uk ([78.32.30.217]:40970 "EHLO pandora.arm.linux.org.uk" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753772AbaIXOxN (ORCPT ); Wed, 24 Sep 2014 10:53:13 -0400 Date: Wed, 24 Sep 2014 15:52:57 +0100 From: Russell King - ARM Linux To: Catalin Marinas Cc: Will Deacon , Nathan Lynch , Daniel Lezcano , Christopher Covington , Doug Anderson , Lorenzo Pieralisi , Marc Zyngier , Mark Rutland , Sonny Rao , Stephen Boyd , Thomas Gleixner , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH v2 0/3] arm_arch_timer: VDSO preparation, code consolidation Message-ID: <20140924145257.GY5182@n2100.arm.linux.org.uk> References: <1411052390-27238-1-git-send-email-nathan_lynch@mentor.com> <20140922153919.GQ25809@arm.com> <20140922223022.GO5182@n2100.arm.linux.org.uk> <20140924144541.GJ15842@e104818-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140924144541.GJ15842@e104818-lin.cambridge.arm.com> User-Agent: Mutt/1.5.19 (2009-01-05) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 24, 2014 at 03:45:41PM +0100, Catalin Marinas wrote: > On Mon, Sep 22, 2014 at 11:30:23PM +0100, Russell King - ARM Linux wrote: > > I raised a while back with Will whether there's much point to having > > this on ARM. While it's useful for virtualisation, the majority of > > 32-bit ARM doesn't run virtualised. > > This has nothing to do with virtualisation. The main reason we use > CNTVCT is to not require kernel binary differences when running the OS > as host or guest. But it does _not_ mean that it is only used when > running as a guest. > > > So there's little point in having the VDSO on the majority of > > platforms - it will just add additional unnecessary cycles slowing > > down the system calls that the VDSO is designed to try to speed up. > > A good reason for VDSO is to avoid a system call for gettimeofday when > you can read the clocks source from user space. That's a significant > improvement on CPUs like A7, A15. I'm *not* arguing against having a VDSO to speed up that crap. What I'm trying to get to the bottom of - something which has been totally lost sight of - is what the friggin effect of this stuff is on CPUs *without* the architected timer. Until I get an answer to what the measured effect is, I'm saying no to VDSO on ARM, because - as seems to be the norm - the evaluation job is only half done. -- FTTC broadband for 0.8mile line: currently at 9.5Mbps down 400kbps up according to speedtest.net.