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Tue, 20 Apr 2021 21:14:12 +0000 Received: from BYAPR12MB3479.namprd12.prod.outlook.com ([fe80::88a:1041:81ed:982]) by BYAPR12MB3479.namprd12.prod.outlook.com ([fe80::88a:1041:81ed:982%7]) with mapi id 15.20.4042.024; Tue, 20 Apr 2021 21:14:12 +0000 X-SNPS-Relay: synopsys.com From: Vineet Gupta To: Arnd Bergmann , Matthew Wilcox CC: Vineet Gupta , "grygorii.strashko@ti.com" , "netdev@vger.kernel.org" , "ilias.apalodimas@linaro.org" , "linux-kernel@vger.kernel.org" , "linux-mips@vger.kernel.org" , "mhocko@kernel.org" , "linux-mm@kvack.org" , "mgorman@suse.de" , "brouer@redhat.com" , "mcroce@linux.microsoft.com" , "linux-snps-arc@lists.infradead.org" , "linuxppc-dev@lists.ozlabs.org" , "hch@lst.de" , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems Thread-Topic: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems Thread-Index: AQHXMxVijWO/awtGlkiVwcCofC+50Kq4AUEAgAS3z4CAAAY0gIAAQlaAgADscoA= Date: Tue, 20 Apr 2021 21:14:12 +0000 Message-ID: <8d0fce1c-be7c-1c9b-bf5c-0c531db496ac@synopsys.com> References: <20210416230724.2519198-1-willy@infradead.org> <20210416230724.2519198-2-willy@infradead.org> <20210417024522.GP2531743@casper.infradead.org> <9f99b0a0-f1c1-f3b0-5f84-3a4bfc711725@synopsys.com> <20210420031029.GI2531743@casper.infradead.org> In-Reply-To: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: user-agent: Mozilla/5.0 (X11; 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charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 4/20/21 12:07 AM, Arnd Bergmann wrote: > On Tue, Apr 20, 2021 at 5:10 AM Matthew Wilcox wrote: >> On Tue, Apr 20, 2021 at 02:48:17AM +0000, Vineet Gupta wrote: >>>> 32-bit architectures which expect 8-byte alignment for 8-byte integers >>>> and need 64-bit DMA addresses (arc, arm, mips, ppc) had their struct >>>> page inadvertently expanded in 2019. >>> FWIW, ARC doesn't require 8 byte alignment for 8 byte integers. This is >>> only needed for 8-byte atomics due to the requirements of LLOCKD/SCOND >>> instructions. >> Ah, like x86? OK, great, I'll drop your arch from the list of >> affected. Thanks! > I mistakenly assumed that i386 and m68k were the only supported > architectures with 32-bit alignment on u64. I checked it now and found > > $ for i in /home/arnd/cross/x86_64/gcc-10.1.0-nolibc/*/bin/*-gcc ; do > echo `echo 'int a = __alignof__(long long);' | $i -xc - -Wall -S -o- | > grep -A1 a: | tail -n 1 | cut -f 3 -d\ ` > ${i#/home/arnd/cross/x86_64/gcc-10.1.0-nolibc/*/bin/} ; done > 8 aarch64-linux-gcc > 8 alpha-linux-gcc > 4 arc-linux-gcc > 8 arm-linux-gnueabi-gcc > 8 c6x-elf-gcc > 4 csky-linux-gcc > 4 h8300-linux-gcc > 8 hppa-linux-gcc > 8 hppa64-linux-gcc > 8 i386-linux-gcc > 8 ia64-linux-gcc > 2 m68k-linux-gcc > 4 microblaze-linux-gcc > 8 mips-linux-gcc > 8 mips64-linux-gcc > 8 nds32le-linux-gcc > 4 nios2-linux-gcc > 4 or1k-linux-gcc > 8 powerpc-linux-gcc > 8 powerpc64-linux-gcc > 8 riscv32-linux-gcc > 8 riscv64-linux-gcc > 8 s390-linux-gcc > 4 sh2-linux-gcc > 4 sh4-linux-gcc > 8 sparc-linux-gcc > 8 sparc64-linux-gcc > 8 x86_64-linux-gcc > 8 xtensa-linux-gcc > > which means that half the 32-bit architectures do this. This may > cause more problems when arc and/or microblaze want to support > 64-bit kernels and compat mode in the future on their latest hardware, > as that means duplicating the x86 specific hacks we have for compat. > > What is alignof(u64) on 64-bit arc? $ echo 'int a = __alignof__(long long);' | arc64-linux-gnu-gcc -xc - -Wall -S -o - | grep -A1 a: | tail -n 1 | cut -f 3 8 Yeah ARCv2 alignment of 4 for 64-bit data was a bit of surprise finding for me as well. When 64-bit load/stores were initially targeted by the internal Metaware compiler (llvm based) they decided to keep alignment to 4 still (granted hardware allowed this) and then gcc guys decided to follow the same ABI. I only found this by accident :-) Can you point me to some specifics on the compat issue. For better of worse, arc64 does''t have a compat 32-bit mode, so everything is 64-on-64 or 32-on-32 (ARC32 flavor of ARCv3) Thx, -Vineet _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel