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=-5.1 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,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 D70ABC636CA for ; Thu, 15 Jul 2021 18:37:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BA8F7613CA for ; Thu, 15 Jul 2021 18:37:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229506AbhGOSkX (ORCPT ); Thu, 15 Jul 2021 14:40:23 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54448 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229518AbhGOSkX (ORCPT ); Thu, 15 Jul 2021 14:40:23 -0400 Received: from pandora.armlinux.org.uk (pandora.armlinux.org.uk [IPv6:2001:4d48:ad52:32c8:5054:ff:fe00:142]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7E46CC06175F; Thu, 15 Jul 2021 11:37:29 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=armlinux.org.uk; s=pandora-2019; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=7prIAEwV0cctqSh3YVeApTqrgra5AsWW5297d2iBdT4=; b=qEmHpWL6yFMcmQxcBuuT/B1bT H88tCTSF4SWBdeodMucSuUcT2RH+tWtYAWzN+SCG6evK3VQqgjQm6/GkZ1JusaoQIaNk8qdKiDB28 fjjIWIrfbRmcN1JNUptbTvSuQaM2CBfjJuDluaLPeQcCHDxTPMgWfySmjUtwEhmehv02l4RGnEaT2 VToSeHoHCqRk4L9Sagcl1ecBfgjwXKLqQ20aOt0E0Vb2FE4C4VBxjzRnw7sQijpHA/xUYMN0QEJpM mp2RS3DgFb9tr1NGEoNs0xIFCsrMOg0sLWldT+y1cMD3Tq6mHlZOCqBRz2ektLeaEVjlXMyXdoGMO DRhRSTKpA==; Received: from shell.armlinux.org.uk ([fd8f:7570:feb6:1:5054:ff:fe00:4ec]:46196) by pandora.armlinux.org.uk with esmtpsa (TLS1.3:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1m46Ei-0001FT-4l; Thu, 15 Jul 2021 19:37:28 +0100 Received: from linux by shell.armlinux.org.uk with local (Exim 4.92) (envelope-from ) id 1m46Eh-0002Ox-U6; Thu, 15 Jul 2021 19:37:27 +0100 Date: Thu, 15 Jul 2021 19:37:27 +0100 From: "Russell King (Oracle)" To: Matthew Wilcox 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: <20210715183727.GP22278@shell.armlinux.org.uk> References: <20210715134612.809280-1-willy@infradead.org> <20210715134612.809280-2-willy@infradead.org> <20210715164740.GN22278@shell.armlinux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: Russell King (Oracle) Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org 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. 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.) 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. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!