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=-15.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 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 A31C7C4338F for ; Wed, 28 Jul 2021 03:09:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 740A960234 for ; Wed, 28 Jul 2021 03:09:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234352AbhG1DJ7 (ORCPT ); Tue, 27 Jul 2021 23:09:59 -0400 Received: from szxga01-in.huawei.com ([45.249.212.187]:7753 "EHLO szxga01-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233437AbhG1DJ6 (ORCPT ); Tue, 27 Jul 2021 23:09:58 -0400 Received: from dggemv704-chm.china.huawei.com (unknown [172.30.72.54]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4GZJQf6y7ZzYhKg; Wed, 28 Jul 2021 11:03:54 +0800 (CST) Received: from dggpemm000001.china.huawei.com (7.185.36.245) by dggemv704-chm.china.huawei.com (10.3.19.47) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Wed, 28 Jul 2021 11:09:48 +0800 Received: from [10.174.177.183] (10.174.177.183) by dggpemm000001.china.huawei.com (7.185.36.245) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Wed, 28 Jul 2021 11:09:47 +0800 Subject: Re: [PATCH -next] riscv: mm: remove redundant trampoline PGD for 64bit To: Alex Ghiti , , , , CC: , , , , References: <20210527144819.12101-1-sunnanyong@huawei.com> <33f93026-cbd2-acd4-e737-c744207f82f2@ghiti.fr> From: "Sunnanyong (Nanyong Sun, Intelligent Computing Solution Development Dep)" Message-ID: Date: Wed, 28 Jul 2021 11:09:47 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <33f93026-cbd2-acd4-e737-c744207f82f2@ghiti.fr> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.177.183] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To dggpemm000001.china.huawei.com (7.185.36.245) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 在 2021/6/29 21:01, Alex Ghiti 写道: > Hi Nanyong, > > Le 27/05/2021 à 16:48, Nanyong Sun a écrit : >> Remove redundant trampoline PGD for 64bit and add more comment >> for why 32bit systems need trampoline PGD. >> >> There was a patch and discussion similar to this,refer to >> the link [1][2]. >> >> The trampoline PGD is redundant for 64bit systems because: >> 1. The early PGD covers the entire kernel mapping. Directly >> loading early PGD can achieve the result in boot stage. >> A more trampoline PGD makes code hard to understand. >> 2. Directly loading early PGD is safe in 64bit systems since >> the kernel virtual address starts as 0xFFFFxxxxxxxxxxxx, >> which has a very big gap with RAM address.It won't fall into >> the corner case that 32bit system worrys. >> 3. Remove redundant trampoline PGD can benefit to code maintaince, >> because 64bit systems have more page table levels.For example: >> If we want to support SV48 which has 4 page table levels, we have >> to add a trampoline_pud and insert it before trampoline_pmd. > > I do agree with your last point about code maintenance and this would > be a welcome improvement before I respin my sv48 series. > > Some comments below though. > >> >> Reference link: >> [1]https://lore.kernel.org/linux-riscv/20190325092234.5451-4-anup.patel@wdc.com/ >> >> [2]https://lkml.org/lkml/2019/3/28/147 >> >> Signed-off-by: Nanyong Sun >> --- >>   arch/riscv/kernel/head.S | 13 +++++++++++-- >>   arch/riscv/mm/init.c     | 21 +++++++-------------- >>   2 files changed, 18 insertions(+), 16 deletions(-) >> >> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S >> index 89cc58ab52b4..1897b17c5fcc 100644 >> --- a/arch/riscv/kernel/head.S >> +++ b/arch/riscv/kernel/head.S >> @@ -93,12 +93,18 @@ relocate: >>       add a2, a2, a1 >>       csrw CSR_TVEC, a2 > > This is not needed in 64-bit then. > > I'm afraid this is still needed for 64-bit, which can convert the physical address world to the virtual address world. I also have made a experiment, it could not boot up if this is deleted. > Thanks, > > Alex > . Hi Alex,     Very thanks for your carefully review, I have updated to version two just now : )