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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 02B0CC636C9 for ; Thu, 15 Jul 2021 19:53:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D7E2E61285 for ; Thu, 15 Jul 2021 19:53:41 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1343544AbhGOT42 (ORCPT ); Thu, 15 Jul 2021 15:56:28 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42300 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1346301AbhGOTvQ (ORCPT ); Thu, 15 Jul 2021 15:51:16 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3ED02C0251B1; Thu, 15 Jul 2021 12:17:54 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=fMdK5kwbGT8Ucs5zmQxr/NzmYbZf7pMhdeOpiEPMJ0A=; b=Tf/modV75F/lM+05pVz6LFMq2e fSW6cVLxK8tmbhYa2Hd6+nfQD0TbDxbu4c4dskRDLhcz/ZYhzpj4yomIRueL06/OutQnuyzccFFU6 kG9qAgrQoj6P5kmH0fI8Rr68nMux2MjqA1PQF7XD8THV40XbKDh6i3DwhJ99CL2dx1Vs6I0gfPshY xB/VwKxt0kDpblWkR6EuAn3bqMHBYip9pn6M+4KUm07JZ/lf48hfLkBjkwa9faBfPBog/nIhj2tX7 6CTMF+V/NYCfQr+I67RNVY4iLyhaVSiN+kdMJoGit6jGK612ZVA+fGppzB4rZDn5TPCN8aK273aPm Wnv1H0jQ==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1m46qE-003ga4-Hi; Thu, 15 Jul 2021 19:16:50 +0000 Date: Thu, 15 Jul 2021 20:16:14 +0100 From: Matthew Wilcox To: "Russell King (Oracle)" Cc: linux-arch@vger.kernel.org, linux-parisc@vger.kernel.org, linux-mips@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/3] arm: Rename PMD_ORDER to PMD_TABLE_ORDER Message-ID: References: <20210715134612.809280-1-willy@infradead.org> <20210715134612.809280-2-willy@infradead.org> <20210715164740.GN22278@shell.armlinux.org.uk> <20210715183727.GP22278@shell.armlinux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210715183727.GP22278@shell.armlinux.org.uk> Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On Thu, Jul 15, 2021 at 07:37:27PM +0100, Russell King (Oracle) wrote: > On Thu, Jul 15, 2021 at 07:10:54PM +0100, Matthew Wilcox wrote: > > On Thu, Jul 15, 2021 at 05:47:41PM +0100, Russell King (Oracle) wrote: > > > On Thu, Jul 15, 2021 at 02:46:10PM +0100, Matthew Wilcox (Oracle) wrote: > > > > This is the order of the page table allocation, not the order of a PMD. > > > > -#define PMD_ORDER 3 > > > > +#define PMD_TABLE_ORDER 3 > > > > #else > > > > #define PG_DIR_SIZE 0x4000 > > > > -#define PMD_ORDER 2 > > > > +#define PMD_TABLE_ORDER 2 > > > > > > I think PMD_ENTRY_ORDER would make more sense here - this is the > > > power-of-2 of an individual PMD entry, not of the entire table. > > > > But ... we have two kinds of PMD entries. We have the direct entry that > > points to a 1-16MB sized chunk of memory, and we have the table entry that > > points to a 4k-32k chunk of memory that contains PTEs. So I don't think > > calling it 'entry' order actually disambiguates anything. That's why > > I went with 'table' -- I can't think of anything else to call it! > > PMD_PTE_ARRAY_ORDER doesn't seem like an improvement to me ... > > There may be two kinds of PMD entries, but that isn't relevant here. > Going back to the original terminology, 1 << PMD_ORDER here is the > size of each PMD entry. It doesn't have anything to do with how much > memory is being mapped by each entry. Oh. Oh! So, 'order' is usually a shift that is _added on to_ the PAGE_SHIFT in order to find how many bytes are in question. See include/asm-generic/getorder.h. Now, PMD_SHIFT is already in use, but perhaps what is meant here is PMD_ENTRY_SHIFT? > I think what is confusing you is stuff like: > > add r0, r4, #KERNEL_OFFSET >> (SECTION_SHIFT - PMD_ORDER) > > r4 is the base address of the page tables, and r0 is the address of > the entry we want to manipulate for "KERNEL_OFFSET" - which is the > virtual address. 1 << SECTION_SHIFT is how much memory each entry > maps (and this is fixed here - there's no variability as you suggest > above.) (the variability I intended above was more to accommodate architectural differences; I hate to use x86-specific numbers like 4KiB and 2MiB) > Effectively, the calculation above is: > > index = KERNEL_OFFSET >> SECTION_SHIFT; > pmd_entry_size = 1 << PMD_ORDER; > r0 = base + index * pmd_entry_size; > > but in a single instruction as we can be sure that KERNEL_OFFSET will > have zeros as the low bits after shifting by SECTION_SHIFT - PMD_ORDER. > > Hope this helps to explain what this PMD_ORDER is actually doing here. Thank you, yes, I was terminally confused.