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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 A2A07C433DF for ; Mon, 18 May 2020 11:37:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 80FAB206D4 for ; Mon, 18 May 2020 11:37:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727882AbgERLhV (ORCPT ); Mon, 18 May 2020 07:37:21 -0400 Received: from foss.arm.com ([217.140.110.172]:38726 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726339AbgERLhS (ORCPT ); Mon, 18 May 2020 07:37:18 -0400 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 73515106F; Mon, 18 May 2020 04:37:18 -0700 (PDT) Received: from C02TD0UTHF1T.local (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id D9F263F52E; Mon, 18 May 2020 04:37:15 -0700 (PDT) Date: Mon, 18 May 2020 12:37:10 +0100 From: Mark Rutland To: Will Deacon Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Sami Tolvanen , Kees Cook , Catalin Marinas , Mark Rutland , Jann Horn , Ard Biesheuvel , Peter Zijlstra , kernel-team@android.com Subject: Re: [PATCH 1/6] arm64: scs: Store absolute SCS stack pointer value in thread_info Message-ID: <20200518113710.GA1957@C02TD0UTHF1T.local> References: <20200515172756.27185-1-will@kernel.org> <20200515172756.27185-2-will@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200515172756.27185-2-will@kernel.org> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 15, 2020 at 06:27:51PM +0100, Will Deacon wrote: > Storing the SCS information in thread_info as a {base,offset} pair > introduces an additional load instruction on the ret-to-user path, > since the SCS stack pointer in x18 has to be converted back to an offset > by subtracting the base. > > Replace the offset with the absolute SCS stack pointer value instead > and avoid the redundant load. > > Signed-off-by: Will Deacon One trivial nit below, but regardless this looks sound to me, and I certainly prefer having the absolute address rather than an offset, so: Reviewed-by: Mark Rutland > diff --git a/kernel/scs.c b/kernel/scs.c > index 9389c28f0853..5ff8663e4a67 100644 > --- a/kernel/scs.c > +++ b/kernel/scs.c > @@ -60,8 +60,7 @@ int scs_prepare(struct task_struct *tsk, int node) > if (!s) > return -ENOMEM; > > - task_scs(tsk) = s; > - task_scs_offset(tsk) = 0; > + task_scs(tsk) = task_scs_sp(tsk) = s; I think this would be more legible as two statements: | task_sys(tsk) = s; | task_scs_sp(tsk) = s; ... as we usually save `foo = bar = baz` stuff for the start of a function or within loop conditions. Thanks, Mark. > scs_account(tsk, 1); > return 0; > } > -- > 2.26.2.761.g0e0b3e54be-goog >