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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 BB709C433DF for ; Wed, 17 Jun 2020 14:45:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A3907208B3 for ; Wed, 17 Jun 2020 14:45:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726594AbgFQOpe (ORCPT ); Wed, 17 Jun 2020 10:45:34 -0400 Received: from pegase1.c-s.fr ([93.17.236.30]:35227 "EHLO pegase1.c-s.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726328AbgFQOpe (ORCPT ); Wed, 17 Jun 2020 10:45:34 -0400 Received: from localhost (mailhub1-int [192.168.12.234]) by localhost (Postfix) with ESMTP id 49n7CX6RNmz9tyTx; Wed, 17 Jun 2020 16:45:28 +0200 (CEST) X-Virus-Scanned: Debian amavisd-new at c-s.fr Received: from pegase1.c-s.fr ([192.168.12.234]) by localhost (pegase1.c-s.fr [192.168.12.234]) (amavisd-new, port 10024) with ESMTP id z61xwnB6Yi86; Wed, 17 Jun 2020 16:45:28 +0200 (CEST) Received: from messagerie.si.c-s.fr (messagerie.si.c-s.fr [192.168.25.192]) by pegase1.c-s.fr (Postfix) with ESMTP id 49n7CX5PZhz9tyTv; Wed, 17 Jun 2020 16:45:28 +0200 (CEST) Received: from localhost (localhost [127.0.0.1]) by messagerie.si.c-s.fr (Postfix) with ESMTP id AC4C08B80F; Wed, 17 Jun 2020 16:45:30 +0200 (CEST) X-Virus-Scanned: amavisd-new at c-s.fr Received: from messagerie.si.c-s.fr ([127.0.0.1]) by localhost (messagerie.si.c-s.fr [127.0.0.1]) (amavisd-new, port 10023) with ESMTP id JE3Mk_gI3482; Wed, 17 Jun 2020 16:45:30 +0200 (CEST) Received: from [192.168.4.90] (unknown [192.168.4.90]) by messagerie.si.c-s.fr (Postfix) with ESMTP id 0C00D8B781; Wed, 17 Jun 2020 16:45:30 +0200 (CEST) Subject: Re: [PATCH 3/3] powerpc/8xx: Provide ptep_get() with 16k pages To: Peter Zijlstra , Michael Ellerman Cc: Benjamin Herrenschmidt , Paul Mackerras , Will Deacon , Andrew Morton , linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org References: <341688399c1b102756046d19ea6ce39db1ae4742.1592225558.git.christophe.leroy@csgroup.eu> <20200615132244.GR2531@hirez.programming.kicks-ass.net> <87wo45db8d.fsf@mpe.ellerman.id.au> <20200617143826.GJ2531@hirez.programming.kicks-ass.net> From: Christophe Leroy Message-ID: <0bb024ce-11aa-80dc-c7d8-d5acc5329f25@csgroup.eu> Date: Wed, 17 Jun 2020 16:45:19 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.9.0 MIME-Version: 1.0 In-Reply-To: <20200617143826.GJ2531@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: fr Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Le 17/06/2020 à 16:38, Peter Zijlstra a écrit : > On Thu, Jun 18, 2020 at 12:21:22AM +1000, Michael Ellerman wrote: >> Peter Zijlstra writes: >>> On Mon, Jun 15, 2020 at 12:57:59PM +0000, Christophe Leroy wrote: > >>>> +#if defined(CONFIG_PPC_8xx) && defined(CONFIG_PPC_16K_PAGES) >>>> +#define __HAVE_ARCH_PTEP_GET >>>> +static inline pte_t ptep_get(pte_t *ptep) >>>> +{ >>>> + pte_t pte = {READ_ONCE(ptep->pte), 0, 0, 0}; >>>> + >>>> + return pte; >>>> +} >>>> +#endif >>> >>> Would it make sense to have a comment with this magic? The casual reader >>> might wonder WTH just happened when he stumbles on this :-) >> >> I tried writing a helpful comment but it's too late for my brain to form >> sensible sentences. >> >> Christophe can you send a follow-up with a comment explaining it? In >> particular the zero entries stand out, it's kind of subtle that those >> entries are only populated with the right value when we write to the >> page table. > > static inline pte_t ptep_get(pte_t *ptep) > { > unsigned long val = READ_ONCE(ptep->pte); > /* 16K pages have 4 identical value 4K entries */ > pte_t pte = {val, val, val, val); > return pte; > } > > Maybe something like that? > This should work as well. Indeed nobody cares about what's in the other three. They are only there to ensure that ptep++ increases the ptep pointer by 16 bytes. Only the HW require 4 identical values, that's taken care of in set_pte_at() and pte_update(). So we should use the most efficient. Thinking once more, maybe what you propose is the most efficient as there is no need to load another register with value 0 in order to write it in the stack. Christophe