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=-5.5 required=3.0 tests=BAYES_00, 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 8BBA3C433E0 for ; Wed, 22 Jul 2020 11:09:58 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 48DD7207CD for ; Wed, 22 Jul 2020 11:09:58 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 48DD7207CD 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 C206D6B0003; Wed, 22 Jul 2020 07:09:57 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id BD1786B0005; Wed, 22 Jul 2020 07:09:57 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id AE6866B0006; Wed, 22 Jul 2020 07:09:57 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0201.hostedemail.com [216.40.44.201]) by kanga.kvack.org (Postfix) with ESMTP id 984E36B0003 for ; Wed, 22 Jul 2020 07:09:57 -0400 (EDT) Received: from smtpin19.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 0D0461809A324 for ; Wed, 22 Jul 2020 11:09:57 +0000 (UTC) X-FDA: 77065442034.19.owner95_1d1638226f35 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin19.hostedemail.com (Postfix) with ESMTP id D6A9714D99 for ; Wed, 22 Jul 2020 11:09:56 +0000 (UTC) X-HE-Tag: owner95_1d1638226f35 X-Filterd-Recvd-Size: 2715 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf16.hostedemail.com (Postfix) with ESMTP for ; Wed, 22 Jul 2020 11:09:56 +0000 (UTC) Received: from gaia (unknown [95.146.230.158]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id AEB55206F5; Wed, 22 Jul 2020 11:09:53 +0000 (UTC) Date: Wed, 22 Jul 2020 12:09:51 +0100 From: Catalin Marinas To: Kevin Brodsky Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, Will Deacon , Dave P Martin , Vincenzo Frascino , Szabolcs Nagy , Andrey Konovalov , Peter Collingbourne , Andrew Morton Subject: Re: [PATCH v7 18/29] arm64: mte: Allow user control of the tag check mode via prctl() Message-ID: <20200722110950.GB27540@gaia> References: <20200715170844.30064-1-catalin.marinas@arm.com> <20200715170844.30064-19-catalin.marinas@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-Rspamd-Queue-Id: D6A9714D99 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam02 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 Mon, Jul 20, 2020 at 04:30:35PM +0100, Kevin Brodsky wrote: > On 15/07/2020 18:08, Catalin Marinas wrote: > > +void mte_thread_switch(struct task_struct *next) > > +{ > > + if (!system_supports_mte()) > > + return; > > + > > + /* avoid expensive SCTLR_EL1 accesses if no change */ > > + if (current->thread.sctlr_tcf0 != next->thread.sctlr_tcf0) > > I think this could be improved by checking whether `next` is a kernel > thread, in which case thread.sctlr_tcf0 is 0 but there is no point in > setting SCTLR_EL1.TCF0, since there should not be any access via TTBR0. It's not about kernel or user thread here. kthread_use_mm() (just use_mm() in older kernels) would set an mm on a kernel thread, temporarily making it behave as a user one. Since the sctlr_tcf0 is per thread, not per mm, we need to switch to the default TCF0 for kthreads so that user accesses (if use_mm() is called) don't generate any tag check faults. Note that switch_mm() does not touch TCF0. If we did allow a global, per-mm TCF0 setting, such kthreads could only handle synchronous faults and no SIGSEGV generated (as we do with copy_{from,to}_user() for normal threads). If we want to revisit per-thread vs per-mm TCF0 setting, now is the time. -- Catalin