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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D78EDC43219 for ; Wed, 2 Mar 2022 11:07:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241241AbiCBLHr (ORCPT ); Wed, 2 Mar 2022 06:07:47 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34000 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236360AbiCBLHq (ORCPT ); Wed, 2 Mar 2022 06:07:46 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 1E91946674; Wed, 2 Mar 2022 03:07:03 -0800 (PST) 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 AAE8C106F; Wed, 2 Mar 2022 03:07:02 -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 417BC3F70D; Wed, 2 Mar 2022 03:06:55 -0800 (PST) Subject: Re: [PATCH V3 09/30] arm/mm: Enable ARCH_HAS_VM_GET_PAGE_PROT To: Geert Uytterhoeven Cc: Christophe Leroy , "Russell King (Oracle)" , "linux-ia64@vger.kernel.org" , "linux-sh@vger.kernel.org" , "linux-mips@vger.kernel.org" , "linux-mm@kvack.org" , "sparclinux@vger.kernel.org" , "linux-riscv@lists.infradead.org" , "linux-arch@vger.kernel.org" , "linux-s390@vger.kernel.org" , "linux-hexagon@vger.kernel.org" , "linux-csky@vger.kernel.org" , Christoph Hellwig , "linux-snps-arc@lists.infradead.org" , "linux-xtensa@linux-xtensa.org" , Arnd Bergmann , "linux-um@lists.infradead.org" , "linux-m68k@lists.linux-m68k.org" , "openrisc@lists.librecores.org" , "linux-arm-kernel@lists.infradead.org" , "linux-parisc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-alpha@vger.kernel.org" , "akpm@linux-foundation.org" , "linuxppc-dev@lists.ozlabs.org" 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> <52866c88-59f9-2d1c-6f5a-5afcaf23f2bb@csgroup.eu> <9caa90f5-c10d-75dd-b403-1388b7a3d296@arm.com> From: Anshuman Khandual Message-ID: Date: Wed, 2 Mar 2022 16:36:52 +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: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-mips@vger.kernel.org On 3/2/22 3:35 PM, Geert Uytterhoeven wrote: > Hi Anshuman, > > On Wed, Mar 2, 2022 at 10:51 AM Anshuman Khandual > wrote: >> On 3/2/22 12:35 PM, Christophe Leroy wrote: >>> Le 02/03/2022 à 04:22, Anshuman Khandual a écrit : >>>> On 3/1/22 1:46 PM, Christophe Leroy wrote: >>>>> Le 01/03/2022 à 01:31, Russell King (Oracle) a écrit : >>>>>> 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. >>>>> >>>>> So do I. >>>>> >>>>>> >>>>>> 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. >>>>> >>>>> On ppc32 I get: >>>>> >>>>> 00000094 : >>>>> 94: 3d 20 00 00 lis r9,0 >>>>> 96: R_PPC_ADDR16_HA .data..ro_after_init >>>>> 98: 54 84 16 ba rlwinm r4,r4,2,26,29 >>>>> 9c: 39 29 00 00 addi r9,r9,0 >>>>> 9e: R_PPC_ADDR16_LO .data..ro_after_init >>>>> a0: 7d 29 20 2e lwzx r9,r9,r4 >>>>> a4: 91 23 00 00 stw r9,0(r3) >>>>> a8: 4e 80 00 20 blr >>>>> >>>>> >>>>>> >>>>>> 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. >>>>> >>>>> With your series I get: >>>>> >>>>> 00000000 : >>>>> 0: 3d 20 00 00 lis r9,0 >>>>> 2: R_PPC_ADDR16_HA .rodata >>>>> 4: 39 29 00 00 addi r9,r9,0 >>>>> 6: R_PPC_ADDR16_LO .rodata >>>>> 8: 54 84 16 ba rlwinm r4,r4,2,26,29 >>>>> c: 7d 49 20 2e lwzx r10,r9,r4 >>>>> 10: 7d 4a 4a 14 add r10,r10,r9 >>>>> 14: 7d 49 03 a6 mtctr r10 >>>>> 18: 4e 80 04 20 bctr >>>>> 1c: 39 20 03 15 li r9,789 >>>>> 20: 91 23 00 00 stw r9,0(r3) >>>>> 24: 4e 80 00 20 blr >>>>> 28: 39 20 01 15 li r9,277 >>>>> 2c: 91 23 00 00 stw r9,0(r3) >>>>> 30: 4e 80 00 20 blr >>>>> 34: 39 20 07 15 li r9,1813 >>>>> 38: 91 23 00 00 stw r9,0(r3) >>>>> 3c: 4e 80 00 20 blr >>>>> 40: 39 20 05 15 li r9,1301 >>>>> 44: 91 23 00 00 stw r9,0(r3) >>>>> 48: 4e 80 00 20 blr >>>>> 4c: 39 20 01 11 li r9,273 >>>>> 50: 4b ff ff d0 b 20 >>>>> >>>>> >>>>> That is definitely more expensive, it implements a table of branches. >>>> >>>> Okay, will split out the PPC32 implementation that retains existing >>>> table look up method. Also planning to keep that inside same file >>>> (arch/powerpc/mm/mmap.c), unless you have a difference preference. >>> >>> My point was not to get something specific for PPC32, but to amplify on >>> Russell's objection. >>> >>> As this is bad for ARM and bad for PPC32, do we have any evidence that >>> your change is good for any other architecture ? >>> >>> I checked PPC64 and there is exactly the same drawback. With the current >>> implementation it is a small function performing table read then a few >>> adjustment. After your change it is a bigger function implementing a >>> table of branches. >> >> I am wondering if this would not be the case for any other switch case >> statement on the platform ? Is there something specific/different just >> on vm_get_page_prot() implementation ? Are you suggesting that switch >> case statements should just be avoided instead ? >> >>> >>> So, as requested by Russell, could you look at the disassembly for other >>> architectures and show us that ARM and POWERPC are the only ones for >>> which your change is not optimal ? >> >> But the primary purpose of this series is not to guarantee optimized >> code on platform by platform basis, while migrating from a table based >> look up method into a switch case statement. >> >> But instead, the purposes is to remove current levels of unnecessary >> abstraction while converting a vm_flags access combination into page >> protection. The switch case statement for platform implementation of >> vm_get_page_prot() just seemed logical enough. Christoph's original >> suggestion patch for x86 had the same implementation as well. >> >> But if the table look up is still better/preferred method on certain >> platforms like arm or ppc32, will be happy to preserve that. > > I doubt the switch() variant would give better code on any platform. > > What about using tables everywhere, using designated initializers > to improve readability? Designated initializers ? Could you please be more specific. A table look up on arm platform would be something like this and arm_protection_map[] needs to be updated with user_pgprot like before. Just wondering how a designated initializer will help here. static pgprot_t arm_protection_map[16] __ro_after_init = { [VM_NONE] = __PAGE_NONE, [VM_READ] = __PAGE_READONLY, [VM_WRITE] = __PAGE_COPY, [VM_WRITE | VM_READ] = __PAGE_COPY, [VM_EXEC] = __PAGE_READONLY_EXEC, [VM_EXEC | VM_READ] = __PAGE_READONLY_EXEC, [VM_EXEC | VM_WRITE] = __PAGE_COPY_EXEC, [VM_EXEC | VM_WRITE | VM_READ] = __PAGE_COPY_EXEC, [VM_SHARED] = __PAGE_NONE, [VM_SHARED | VM_READ] = __PAGE_READONLY, [VM_SHARED | VM_WRITE] = __PAGE_SHARED, [VM_SHARED | VM_WRITE | VM_READ] = __PAGE_SHARED, [VM_SHARED | VM_EXEC] = __PAGE_READONLY_EXEC, [VM_SHARED | VM_EXEC | VM_READ] = __PAGE_READONLY_EXEC, [VM_SHARED | VM_EXEC | VM_WRITE] = __PAGE_SHARED_EXEC, [VM_SHARED | VM_EXEC | VM_WRITE | VM_READ] = __PAGE_SHARED_EXEC }; pgprot_t vm_get_page_prot(unsigned long vm_flags) { return __pgprot(pgprot_val(arm_protection_map[vm_flags & (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)])); } EXPORT_SYMBOL(vm_get_page_prot);