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 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 24AE6C433EF for ; Wed, 2 Mar 2022 03:16:05 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 7659D8D0002; Tue, 1 Mar 2022 22:16:04 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 715338D0001; Tue, 1 Mar 2022 22:16:04 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 604598D0002; Tue, 1 Mar 2022 22:16:04 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0165.hostedemail.com [216.40.44.165]) by kanga.kvack.org (Postfix) with ESMTP id 5274B8D0001 for ; Tue, 1 Mar 2022 22:16:04 -0500 (EST) Received: from smtpin30.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id F1A6C8248D52 for ; Wed, 2 Mar 2022 03:16:03 +0000 (UTC) X-FDA: 79197982206.30.DA26D94 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by imf01.hostedemail.com (Postfix) with ESMTP id 37DE94000A for ; Wed, 2 Mar 2022 03:16:03 +0000 (UTC) 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 2C1C9113E; Tue, 1 Mar 2022 19:16:02 -0800 (PST) Received: from [10.163.49.202] (unknown [10.163.49.202]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 091233F73D; Tue, 1 Mar 2022 19:15:54 -0800 (PST) Subject: Re: [PATCH V3 09/30] arm/mm: Enable ARCH_HAS_VM_GET_PAGE_PROT To: "Russell King (Oracle)" Cc: linux-mm@kvack.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, geert@linux-m68k.org, Christoph Hellwig , linuxppc-dev@lists.ozlabs.org, linux-arm-kernel@lists.infradead.org, sparclinux@vger.kernel.org, linux-mips@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-s390@vger.kernel.org, linux-riscv@lists.infradead.org, linux-alpha@vger.kernel.org, linux-sh@vger.kernel.org, linux-snps-arc@lists.infradead.org, linux-csky@vger.kernel.org, linux-xtensa@linux-xtensa.org, linux-parisc@vger.kernel.org, openrisc@lists.librecores.org, linux-um@lists.infradead.org, linux-hexagon@vger.kernel.org, linux-ia64@vger.kernel.org, linux-arch@vger.kernel.org, Arnd Bergmann References: <1646045273-9343-1-git-send-email-anshuman.khandual@arm.com> <1646045273-9343-10-git-send-email-anshuman.khandual@arm.com> <542fa048-131e-240b-cc3a-fd4fff7ce4ba@arm.com> From: Anshuman Khandual Message-ID: <2f995e2a-c00f-bc9c-6ac6-c783a21b007e@arm.com> Date: Wed, 2 Mar 2022 08:45:52 +0530 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam10 X-Rspam-User: X-Stat-Signature: w8p3zdnmswwy1g5ru6aunp31wuoiwsii Authentication-Results: imf01.hostedemail.com; dkim=none; spf=pass (imf01.hostedemail.com: domain of anshuman.khandual@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=anshuman.khandual@arm.com; dmarc=pass (policy=none) header.from=arm.com X-Rspamd-Queue-Id: 37DE94000A X-HE-Tag: 1646190963-107150 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000004, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 3/1/22 6:01 AM, Russell King (Oracle) wrote: > On Tue, Mar 01, 2022 at 05:30:41AM +0530, Anshuman Khandual wrote: >> On 2/28/22 4:27 PM, Russell King (Oracle) wrote: >>> On Mon, Feb 28, 2022 at 04:17:32PM +0530, Anshuman Khandual wrote: >>>> This defines and exports a platform specific custom vm_get_page_prot() via >>>> subscribing ARCH_HAS_VM_GET_PAGE_PROT. Subsequently all __SXXX and __PXXX >>>> macros can be dropped which are no longer needed. >>> What I would really like to know is why having to run _code_ to work out >>> what the page protections need to be is better than looking it up in a >>> table. >>> >>> Not only is this more expensive in terms of CPU cycles, it also brings >>> additional code size with it. >>> >>> I'm struggling to see what the benefit is. >> Currently vm_get_page_prot() is also being _run_ to fetch required page >> protection values. Although that is being run in the core MM and from a >> platform perspective __SXXX, __PXXX are just being exported for a table. >> Looking it up in a table (and applying more constructs there after) is >> not much different than a clean switch case statement in terms of CPU >> usage. So this is not more expensive in terms of CPU cycles. > I disagree. > > However, let's base this disagreement on some evidence. Here is the > present 32-bit ARM implementation: > > 00000048 : > 48: e200000f and r0, r0, #15 > 4c: e3003000 movw r3, #0 > 4c: R_ARM_MOVW_ABS_NC .LANCHOR1 > 50: e3403000 movt r3, #0 > 50: R_ARM_MOVT_ABS .LANCHOR1 > 54: e7930100 ldr r0, [r3, r0, lsl #2] > 58: e12fff1e bx lr > > That is five instructions long. > > Please show that your new implementation is not more expensive on > 32-bit ARM. Please do so by building a 32-bit kernel, and providing > the disassembly. > > I think you will find way more than five instructions in your version - > the compiler will have to issue code to decode the protection bits, > probably using a table of branches or absolute PC values, or possibly > the worst case of using multiple comparisons and branches. It then has > to load constants that may be moved using movw on ARMv7, but on > older architectures would have to be created from multiple instructions > or loaded from the literal pool. Then there'll be instructions to load > the address of "user_pgprot", retrieve its value, and bitwise or that. > > Therefore, I fail to see how your approach of getting rid of the table > is somehow "better" than what we currently have in terms of the effect > on the resulting code. > > If you don't like the __P and __S stuff and two arch_* hooks, you could > move the table into arch code along with vm_get_page_prot() without the > additional unnecessary hooks, while keeping all the benefits of the > table lookup. Okay, will change the arm's vm_get_page_prot() implementation as suggested.