From mboxrd@z Thu Jan 1 00:00:00 1970 From: Patrick Daly Subject: Re: [PATCH v4 24/26] arm64: mte: Introduce early param to disable MTE support Date: Thu, 21 May 2020 22:57:10 -0700 Message-ID: <20200522055710.GA25791@pdaly-linux.qualcomm.com> References: <20200515171612.1020-1-catalin.marinas@arm.com> <20200515171612.1020-25-catalin.marinas@arm.com> <20200518113103.GA32394@willie-the-truck> <20200518172054.GL9862@gaia> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mail27.static.mailgun.info ([104.130.122.27]:27416 "EHLO mail27.static.mailgun.info" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725894AbgEVF5T (ORCPT ); Fri, 22 May 2020 01:57:19 -0400 Content-Disposition: inline In-Reply-To: <20200518172054.GL9862@gaia> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Catalin Marinas Cc: Will Deacon , linux-arch@vger.kernel.org, Vladimir Murzin , Szabolcs Nagy , Andrey Konovalov , Kevin Brodsky , linux-mm@kvack.org, Vincenzo Frascino , Peter Collingbourne , Dave P Martin , linux-arm-kernel@lists.infradead.org On Mon, May 18, 2020 at 06:20:55PM +0100, Catalin Marinas wrote: > On Mon, May 18, 2020 at 12:31:03PM +0100, Will Deacon wrote: > > On Mon, May 18, 2020 at 12:26:30PM +0100, Vladimir Murzin wrote: > > > On 5/15/20 6:16 PM, Catalin Marinas wrote: > > > > For performance analysis it may be desirable to disable MTE altogether > > > > via an early param. Introduce arm64.mte_disable and, if true, filter out > > > > the sanitised ID_AA64PFR1_EL1.MTE field to avoid exposing the HWCAP to > > > > user. > > > > > > > > Signed-off-by: Catalin Marinas > > > > Cc: Will Deacon > > > > --- > > > > > > > > Notes: > > > > New in v4. > > > > > > > > Documentation/admin-guide/kernel-parameters.txt | 4 ++++ > > > > arch/arm64/kernel/cpufeature.c | 11 +++++++++++ > > > > 2 files changed, 15 insertions(+) > > > > > > > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > > > > index f2a93c8679e8..7436e7462b85 100644 > > > > --- a/Documentation/admin-guide/kernel-parameters.txt > > > > +++ b/Documentation/admin-guide/kernel-parameters.txt > > > > @@ -373,6 +373,10 @@ > > > > arcrimi= [HW,NET] ARCnet - "RIM I" (entirely mem-mapped) cards > > > > Format: ,, > > > > > > > > + arm64.mte_disable= > > > > + [ARM64] Disable Linux support for the Memory > > > > + Tagging Extension (both user and in-kernel). > > > > + > > > > > > Should it really to take parameter (on/off/true/false)? It may lead to expectation > > > that arm64.mte_disable=false should enable MT and, yes, double negatives make it > > > look ugly, so if we do need parameter, can it be arm64.mte=on/off/true/false? > > > > I don't think "performance analysis" is a good justification for this > > parameter tbh. We don't tend to add these options for other architectural > > features, and I don't see why MTE is any different in this regard. > > There is an expectation of performance impact with MTE enabled, > especially if it's running in synchronous mode. For the in-kernel MTE, > we could add a parameter which sets sync vs async at boot time rather > than a big disable knob. It won't affect user space however. > > The other 'justification' is if your hardware has weird unexpected > behaviour but I'd like this handled via errata workarounds. > > I'll let the people who asked for this to chip in ;). I agree with you > that we rarely add these (and I rejected a similar option a few weeks > ago on the AMU patchset). We've been looking into other ways this on/off behavior could be achieved. The "arm,armv8.5-memtag" DT flag already provides what we want - meaning that this flag could be removed if the system did not support MTE. I did see your remark on "arm64: mte: Check the DT memory nodes for MTE support" questioning whether it was the right approach - is this still the case? --Patrick -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project