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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 6EBF7C433DF for ; Fri, 15 May 2020 10:38:50 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 2E9922065C for ; Fri, 15 May 2020 10:38:50 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2E9922065C Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id AE86990000C; Fri, 15 May 2020 06:38:49 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id A996A8E0005; Fri, 15 May 2020 06:38:49 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 986FC90000C; Fri, 15 May 2020 06:38:49 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0049.hostedemail.com [216.40.44.49]) by kanga.kvack.org (Postfix) with ESMTP id 80D3B8E0005 for ; Fri, 15 May 2020 06:38:49 -0400 (EDT) Received: from smtpin04.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id 1847140C7 for ; Fri, 15 May 2020 10:38:49 +0000 (UTC) X-FDA: 76818605178.04.stop42_12b84ad22f917 X-HE-Tag: stop42_12b84ad22f917 X-Filterd-Recvd-Size: 4173 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf47.hostedemail.com (Postfix) with ESMTP for ; Fri, 15 May 2020 10:38:48 +0000 (UTC) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id BF24B2F; Fri, 15 May 2020 03:38:47 -0700 (PDT) Received: from gaia (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3B4033F71E; Fri, 15 May 2020 03:38:46 -0700 (PDT) Date: Fri, 15 May 2020 11:38:40 +0100 From: Catalin Marinas To: Dave Martin Cc: linux-arch@vger.kernel.org, Richard Earnshaw , Szabolcs Nagy , Andrey Konovalov , Kevin Brodsky , Peter Collingbourne , linux-mm@kvack.org, Vincenzo Frascino , Will Deacon , linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v3 23/23] arm64: mte: Add Memory Tagging Extension documentation Message-ID: <20200515103839.GA22393@gaia> References: <20200421142603.3894-1-catalin.marinas@arm.com> <20200421142603.3894-24-catalin.marinas@arm.com> <20200429164705.GF30377@arm.com> <20200430162316.GJ2717@gaia> <20200504164617.GK30377@arm.com> <20200511164018.GC19176@gaia> <20200513154845.GT21779@arm.com> <20200514113722.GA1907@gaia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200514113722.GA1907@gaia> User-Agent: Mutt/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, May 14, 2020 at 12:37:22PM +0100, Catalin Marinas wrote: > On Wed, May 13, 2020 at 04:48:46PM +0100, Dave P Martin wrote: > > > > > On Wed, Apr 29, 2020 at 05:47:05PM +0100, Dave P Martin wrote: > > > > > > On Tue, Apr 21, 2020 at 03:26:03PM +0100, Catalin Marinas wrote: > > > > > > > +excludes all tags other than 0. A user thread can enable specific tags > > > > > > > +in the randomly generated set using the ``prctl(PR_SET_TAGGED_ADDR_CTRL, > > > > > > > +flags, 0, 0, 0)`` system call where ``flags`` contains the tags bitmap > > > > > > > +in the ``PR_MTE_TAG_MASK`` bit-field. > > > > > > > + > > > > > > > +**Note**: The hardware uses an exclude mask but the ``prctl()`` > > > > > > > +interface provides an include mask. An include mask of ``0`` (exclusion > > > > > > > +mask ``0xffff``) results in the CPU always generating tag ``0``. > > > > > > > > > > > > Is there no way to make this default to 1 rather than having a magic > > > > > > meaning for 0? > [...] > > The only configuration that doesn't make sense is "no tags allowed", so > > I'd argue for explicity blocking that, even if the architeture aliases > > that encoding to something else. > > > > If we prefer 0 as a default value so that init inherits the correct > > value from the kernel without any special acrobatics, then we make it an > > exclude mask, with the semantics that the hardware is allowed to > > generate any of these tags, but does not have to be capable of > > generating all of them. > > That's more of a question to the libc people and their preference. > We have two options with suboptions: > > 1. prctl() gets an exclude mask with 0xffff illegal even though the > hardware accepts it: > a) default exclude mask 0, allowing all tags to be generated by IRG > b) default exclude mask of 0xfffe so that only tag 0 is generated > > 2. prctl() gets an include mask with 0 illegal: > a) default include mask is 0xffff, allowing all tags to be generated > b) default include mask 0f 0x0001 so that only tag 0 is generated > > We currently have (2) with mask 0 but could be changed to (2.b). If we > are to follow the hardware description (which makes more sense to me but > I don't write the C library), (1.a) is the most appropriate. Thinking some more about this, as we are to expose the GCR_EL1.Excl via a ptrace interface as a regset, it makes more sense to move back to an exclude mask here with default 0. That would be option 1.a above. -- Catalin