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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id B29DFC433EF for ; Wed, 2 Mar 2022 03:16:34 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:Date: Message-ID:From:References:Cc:To:Subject:Reply-To:Content-ID: Content-Description:Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc :Resent-Message-ID:List-Owner; bh=FYqjt9cBC7hNPwt/gV018eFqoETnHAqR/S6kqoEdge4=; b=UAUl/7tsHaKwOc7JgzEr8YWMCL 00nD5a9Ny5OmgqPnTVF90mH+odO6q8omDd3JObDrC6KNi4OB98JaBRgzG+pxtnfIMYaIib04kFAKO rDAxxqAAEPLZlGl6EaK0mojJ5EcrJukRtBrXcVKIJr87gfSSmhqFvAQFLVdZPCgyzRRJ40nR/dAbq tR7w2e90wPL9JLCc6FItBC/bFgoogNvgA5eC67erAgngeg/1gaD6rvjcPJDkyntEVgb3Bni1MbNuD R76XbXAO8meaLyXJ6R4/GwAVq0BE80oFzlwqfKMbvH3uRy8L7F4akmdQcL9FDyBQU7uQirE/MFlqu ShaXBr9w==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1nPFTT-001GvU-PK; Wed, 02 Mar 2022 03:16:23 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1nPFTB-001Gnf-Dl; Wed, 02 Mar 2022 03:16:08 +0000 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-Language: en-US X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20220301_191605_613694_8A92CB30 X-CRM114-Status: GOOD ( 27.03 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org 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. _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv