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 E71CAC4CEC4 for ; Wed, 18 Sep 2019 16:49:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B9BE6218AE for ; Wed, 18 Sep 2019 16:49:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732028AbfIRQtb (ORCPT ); Wed, 18 Sep 2019 12:49:31 -0400 Received: from foss.arm.com ([217.140.110.172]:45132 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726559AbfIRQtb (ORCPT ); Wed, 18 Sep 2019 12:49:31 -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 A0961337; Wed, 18 Sep 2019 09:49:30 -0700 (PDT) Received: from arrakis.emea.arm.com (arrakis.cambridge.arm.com [10.1.196.78]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id F0AAA3F59C; Wed, 18 Sep 2019 09:49:27 -0700 (PDT) Date: Wed, 18 Sep 2019 17:49:25 +0100 From: Catalin Marinas To: Matthew Wilcox Cc: Jia He , Will Deacon , Mark Rutland , James Morse , Marc Zyngier , "Kirill A. Shutemov" , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Suzuki Poulose , Punit Agrawal , Anshuman Khandual , Jun Yao , Alex Van Brunt , Robin Murphy , Thomas Gleixner , Andrew Morton , =?iso-8859-1?B?Suly9G1l?= Glisse , Ralph Campbell , hejianet@gmail.com, Kaly Xin Subject: Re: [PATCH v4 1/3] arm64: cpufeature: introduce helper cpu_has_hw_af() Message-ID: <20190918164925.GB41588@arrakis.emea.arm.com> References: <20190918131914.38081-1-justin.he@arm.com> <20190918131914.38081-2-justin.he@arm.com> <20190918142017.GC9880@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190918142017.GC9880@bombadil.infradead.org> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 18, 2019 at 07:20:17AM -0700, Matthew Wilcox wrote: > On Wed, Sep 18, 2019 at 09:19:12PM +0800, Jia He wrote: > > +/* Decouple AF from AFDBM. */ > > +bool cpu_has_hw_af(void) > > +{ > > + return (read_cpuid(ID_AA64MMFR1_EL1) & 0xf); > > +} > > + > > Do you really want to call read_cpuid() every time? I would have thought > you'd want to use the static branch mechanism to do the right thing at > boot time. See Documentation/static-keys.txt. We do have a static branch mechanism for other CPU features but mostly because on some big.LITTLE systems, the CPUs may differ slightly so we only advertise the common features. I guess here the additional instructions for reading and checking the CPUID here would be lost in the noise compared to the actual page copying. -- Catalin