From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755668AbdGXMUC (ORCPT ); Mon, 24 Jul 2017 08:20:02 -0400 Received: from foss.arm.com ([217.140.101.70]:60430 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753334AbdGXMTx (ORCPT ); Mon, 24 Jul 2017 08:19:53 -0400 Date: Mon, 24 Jul 2017 13:19:59 +0100 From: Will Deacon To: Marc Zyngier Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Mark Rutland , Russell King , Catalin Marinas , Daniel Lezcano , Thomas Gleixner Subject: Re: [PATCH 00/16] arm/arm64: Workaround misprogrammed CNTFRQ Message-ID: <20170724121958.GC29919@arm.com> References: <20170721171542.27639-1-marc.zyngier@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170721171542.27639-1-marc.zyngier@arm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 21, 2017 at 06:15:26PM +0100, Marc Zyngier wrote: > It is an unfortunate situation that CNTFRQ{,_EL0} is often > misprogrammed from the firmware side, leaving it up to the kernel to > work around it. This is usually done by providing an alternative > frequency in the Device Tree. > > Unfortunately, CNTFRQ is accessible from EL0, giving userspace the > wrong frequency, and potentially a different frequency per CPU, which > is definitely not what you want. A possible workaround is to trap this > into the kernel and to emulate it (together with the VDSO being > disabled), and this is what this series is achieving. Which userspace is actually affected by a broken CNTFRQ register? I suspect most users will be more upset at losing their (perfectly functional) vDSO acceleration than they are about having a broken CNTFRQ value that is hardly ever used, especially since this affects quite a few systems. Will