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,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,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 3BDF8C433E0 for ; Wed, 22 Jul 2020 11:11:49 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id F0DEF207CD for ; Wed, 22 Jul 2020 11:11:48 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="RUw2SrHr" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org F0DEF207CD Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References:Message-ID: Subject:To:From:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=LsHVbLR09Btkk5ksfnepn030B0iQGQmeB2FcchHdmJ8=; b=RUw2SrHru9rhocp2jT9keTtwV Aec7erfEl6IhMEHAMFd/8pjGSYYy+X3e0kFPvIcjChHy5dKlwhiLiRkkKnJXpc4u7PjB1Flg8E6BI PpMNcQ8fQ+7XjqH9DKLrfakXppQZ21RCimr5aeR1sEAsa4LAZGnXKrD1zOGtHMQul2gZpvLi7Do+l Ck+rdQTiy7IDaxOOGpw6l6DlefHl7UOp+D3QIuzujedwbdBreKp3QkU3lBybsqdxZ2t69s38e5IGC Nncz0EPhib4EgSJJtEIAal+nVuDu0hTU0hEBjljcSl1H5dLQRgIWuTGeW+O8x4zhIw5mTuhnBqlEz tVMKTBTkg==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jyCdX-0005jS-JR; Wed, 22 Jul 2020 11:10:11 +0000 Received: from mail.kernel.org ([198.145.29.99]) by merlin.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1jyCdI-0005cg-Du for linux-arm-kernel@lists.infradead.org; Wed, 22 Jul 2020 11:10:02 +0000 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 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-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200722_070956_623479_42624351 X-CRM114-Status: GOOD ( 15.00 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch@vger.kernel.org, Szabolcs Nagy , Andrey Konovalov , Peter Collingbourne , linux-mm@kvack.org, Andrew Morton , Vincenzo Frascino , Will Deacon , Dave P Martin , linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel