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 278B6C433DF for ; Mon, 18 May 2020 16:52:30 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 0A20F20674 for ; Mon, 18 May 2020 16:52:29 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0A20F20674 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 A5415900003; Mon, 18 May 2020 12:52:28 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id A0531900002; Mon, 18 May 2020 12:52:28 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 91B74900003; Mon, 18 May 2020 12:52:28 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0144.hostedemail.com [216.40.44.144]) by kanga.kvack.org (Postfix) with ESMTP id 7B311900002 for ; Mon, 18 May 2020 12:52:28 -0400 (EDT) Received: from smtpin24.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 38E9E629 for ; Mon, 18 May 2020 16:52:28 +0000 (UTC) X-FDA: 76830433176.24.spade13_6cd8ddc084f15 X-HE-Tag: spade13_6cd8ddc084f15 X-Filterd-Recvd-Size: 3657 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf14.hostedemail.com (Postfix) with ESMTP for ; Mon, 18 May 2020 16:52:27 +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 1A1D3106F; Mon, 18 May 2020 09:52:27 -0700 (PDT) Received: from arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7EE723F305; Mon, 18 May 2020 09:52:25 -0700 (PDT) Date: Mon, 18 May 2020 17:52:23 +0100 From: Dave Martin To: Szabolcs Nagy Cc: Catalin Marinas , linux-arch@vger.kernel.org, Richard Earnshaw , nd@arm.com, Will Deacon , Andrey Konovalov , Kevin Brodsky , linux-mm@kvack.org, Vincenzo Frascino , Peter Collingbourne , linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v3 23/23] arm64: mte: Add Memory Tagging Extension documentation Message-ID: <20200518165223.GB5031@arm.com> References: <20200504164617.GK30377@arm.com> <20200511164018.GC19176@gaia> <20200513154845.GT21779@arm.com> <20200514113722.GA1907@gaia> <20200515103839.GA22393@gaia> <20200515111359.GC27289@arm.com> <20200515112740.GB22393@gaia> <20200515120433.GE27289@arm.com> <20200515121343.GC22393@gaia> <20200515125332.GF27289@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200515125332.GF27289@arm.com> User-Agent: Mutt/1.5.23 (2014-03-12) 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 01:53:32PM +0100, Szabolcs Nagy wrote: > The 05/15/2020 13:13, Catalin Marinas wrote: > > On Fri, May 15, 2020 at 01:04:33PM +0100, Szabolcs Nagy wrote: > > > The 05/15/2020 12:27, Catalin Marinas wrote: > > > > 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 > > > > > > it seems RRND=1 is the impl specific algorithm. > > > > Yes, that's the implementation specific algorithm which shouldn't be > > worse than the standard one. > > > > > > 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. > > > > > > i'm not familiar with how irg works. > > > > It generates a random tag based on some algorithm. > > > > > is the seed per process state that's set up at process startup in some > > > way? or shared (and thus effectively irg is non-deterministic in > > > userspace)? > > > > The seed is only relevant if the standard algorithm is used (RRND=0). > > i wanted to understand if we can get deterministic > irg behaviour in user space (which may be useful > for debugging to get reproducible tag failures). > > i guess if no control is exposed that means non- > deterministic irg. i think this is fine. Hmmm, I guess this might eventually be wanted. But it's probably OK not to have it to begin with. Things like CRIU restores won't be reproducible unless the seeds can be saved/restored. Doesn't seem essential from day 1 though. Cheers ---Dave