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 C2F3AC433E0 for ; Fri, 15 May 2020 11:27:50 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 8CF2D2065C for ; Fri, 15 May 2020 11:27:50 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 8CF2D2065C 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 1748590000C; Fri, 15 May 2020 07:27:50 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 1266D8E0005; Fri, 15 May 2020 07:27:50 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 03BF790000C; Fri, 15 May 2020 07:27:49 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0104.hostedemail.com [216.40.44.104]) by kanga.kvack.org (Postfix) with ESMTP id DBECB8E0005 for ; Fri, 15 May 2020 07:27:49 -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 9CA703CEA for ; Fri, 15 May 2020 11:27:49 +0000 (UTC) X-FDA: 76818728658.04.drum92_a10ef1271803 X-HE-Tag: drum92_a10ef1271803 X-Filterd-Recvd-Size: 4001 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf13.hostedemail.com (Postfix) with ESMTP for ; Fri, 15 May 2020 11:27:49 +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 56834D6E; Fri, 15 May 2020 04:27:48 -0700 (PDT) Received: from gaia (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id AD4733F305; Fri, 15 May 2020 04:27:46 -0700 (PDT) Date: Fri, 15 May 2020 12:27:40 +0100 From: Catalin Marinas To: Szabolcs Nagy Cc: Dave Martin , linux-arch@vger.kernel.org, Richard Earnshaw , Andrey Konovalov , Kevin Brodsky , Peter Collingbourne , linux-mm@kvack.org, Vincenzo Frascino , Will Deacon , linux-arm-kernel@lists.infradead.org, nd@arm.com Subject: Re: [PATCH v3 23/23] arm64: mte: Add Memory Tagging Extension documentation Message-ID: <20200515112740.GB22393@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> <20200515103839.GA22393@gaia> <20200515111359.GC27289@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200515111359.GC27289@arm.com> 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 Fri, May 15, 2020 at 12:14:00PM +0100, Szabolcs Nagy wrote: > The 05/15/2020 11:38, Catalin Marinas wrote: > > On Thu, May 14, 2020 at 12:37:22PM +0100, Catalin Marinas wrote: > > > 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. > > i think the libc has to do a prctl call to set > mte up and at that point it will use whatever > arguments necessary, so 1.a should work (just > like the other options). > > likely libc will disable 0 for irg and possibly > one or two other fixed colors (which will have > specific use). > > the difference i see between 1 vs 2 is forward > compatibility if the architecture changes (e.g. > adding more tag bits) but then likely new prctl > flag will be needed for handling that so it's > probably not an issue. Thanks Szabolcs. While we are at this, no-one so far asked for the GCR_EL1.RRND to be exposed to user (and this implies RGSR_EL1.SEED). Since RRND=1 guarantees a distribution "no worse" than that of RRND=0, I thought there isn't much point in exposing this configuration to the user. The only advantage of RRND=0 I see is that the kernel can change the seed randomly but, with only 4 bits per tag, it really doesn't matter much. Anyway, mentioning it here in case anyone is surprised later about the lack of RRND configurability. -- Catalin