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[85.187.216.236]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-b7654d55d7fsm1292288866b.21.2025.11.24.05.24.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 24 Nov 2025 05:24:42 -0800 (PST) Message-ID: <3e191e87-5b7f-49e9-b794-eb244d478c56@suse.com> Date: Mon, 24 Nov 2025 15:24:40 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] RAS/AMD/ATL: Remove bitwise_xor_bits To: Borislav Petkov , Kuan-Wei Chiu Cc: linux-edac@vger.kernel.org, linux-kernel@vger.kernel.org, Yazen.Ghannam@amd.com References: <20251124084011.1575166-1-nik.borisov@suse.com> <20251124110526.GAaSQ79mo0yx1h1Xxm@fat_crate.local> <20251124125249.GDaSRVIapy2dmis28p@fat_crate.local> Content-Language: en-US From: Nikolay Borisov Autocrypt: addr=nik.borisov@suse.com; keydata= xsFNBGcrpvIBEAD5cAR5+qu30GnmPrK9veWX5RVzzbgtkk9C/EESHy9Yz0+HWgCVRoNyRQsZ 7DW7vE1KhioDLXjDmeu8/0A8u5nFMqv6d1Gt1lb7XzSAYw7uSWXLPEjFBtz9+fBJJLgbYU7G OpTKy6gRr6GaItZze+r04PGWjeyVUuHZuncTO7B2huxcwIk9tFtRX21gVSOOC96HcxSVVA7X N/LLM2EOL7kg4/yDWEhAdLQDChswhmdpHkp5g6ytj9TM8bNlq9I41hl/3cBEeAkxtb/eS5YR 88LBb/2FkcGnhxkGJPNB+4Siku7K8Mk2Y6elnkOctJcDvk29DajYbQnnW4nhfelZuLNupb1O M0912EvzOVI0dIVgR+xtosp66bYTOpX4Xb0fylED9kYGiuEAeoQZaDQ2eICDcHPiaLzh+6cc pkVTB0sXkWHUsPamtPum6/PgWLE9vGI5s+FaqBaqBYDKyvtJfLK4BdZng0Uc3ijycPs3bpbQ bOnK9LD8TYmYaeTenoNILQ7Ut54CCEXkP446skUMKrEo/HabvkykyWqWiIE/UlAYAx9+Ckho TT1d2QsmsAiYYWwjU8igXBecIbC0uRtF/cTfelNGrQwbICUT6kJjcOTpQDaVyIgRSlUMrlNZ XPVEQ6Zq3/aENA8ObhFxE5PLJPizJH6SC89BMKF3zg6SKx0qzQARAQABzSZOaWtvbGF5IEJv cmlzb3YgPG5pay5ib3Jpc292QHN1c2UuY29tPsLBkQQTAQoAOxYhBDuWB8EJLBUZCPjT3SRn XZEnyhfsBQJnK6byAhsDBQsJCAcCAiICBhUKCQgLAgQWAgMBAh4HAheAAAoJECRnXZEnyhfs XbIQAJxuUnelGdXbSbtovBNm+HF3LtT0XnZ0+DoR0DemUGuA1bZAlaOXGr5mvVbTgaoGUQIJ 3Ejx3UBEG7ZSJcfJobB34w1qHEDO0pN9orGIFT9Bic3lqhawD2r85QMcWwjsZH5FhyRx7P2o DTuUClLMO95GuHYQngBF2rHHl8QMJPVKsR18w4IWAhALpEApxa3luyV7pAAqKllfCNt7tmed uKmclf/Sz6qoP75CvEtRbfAOqYgG1Uk9A62C51iAPe35neMre3WGLsdgyMj4/15jPYi+tOUX Tc7AAWgc95LXyPJo8069MOU73htZmgH4OYy+S7f+ArXD7h8lTLT1niff2bCPi6eiAQq6b5CJ Ka4/27IiZo8tm1XjLYmoBmaCovqx5y5Xt2koibIWG3ZGD2I+qRwZ0UohKRH6kKVHGcrmCv0J YO8yIprxgoYmA7gq21BpTqw3D4+8xujn/6LgndLKmGESM1FuY3ymXgj5983eqaxicKpT9iq8 /a1j31tms4azR7+6Dt8H4SagfN6VbJ0luPzobrrNFxUgpjR4ZyQQ++G7oSRdwjfIh1wuCF6/ mDUNcb6/kA0JS9otiC3omfht47yQnvod+MxFk1lTNUu3hePJUwg1vT1te3vO5oln8lkUo9BU knlYpQ7QA2rDEKs+YWqUstr4pDtHzwQ6mo0rqP+zzsFNBGcrpvIBEADGYTFkNVttZkt6e7yA LNkv3Q39zQCt8qe7qkPdlj3CqygVXfw+h7GlcT9fuc4kd7YxFys4/Wd9icj9ZatGMwffONmi LnUotIq2N7+xvc4Xu76wv+QJpiuGEfCDB+VdZOmOzUPlmMkcJc/EDSH4qGogIYRu72uweKEq VfBI43PZIGpGJ7TjS3THX5WVI2YNSmuwqxnQF/iVqDtD2N72ObkBwIf9GnrOgxEyJ/SQq2R0 g7hd6IYk7SOKt1a8ZGCN6hXXKzmM6gHRC8fyWeTqJcK4BKSdX8PzEuYmAJjSfx4w6DoxdK5/ 9sVrNzaVgDHS0ThH/5kNkZ65KNR7K2nk45LT5Crjbg7w5/kKDY6/XiXDx7v/BOR/a+Ryo+lM MffN3XSnAex8cmIhNINl5Z8CAvDLUtItLcbDOv7hdXt6DSyb65CdyY8JwOt6CWno1tdjyDEG 5ANwVPYY878IFkOJLRTJuUd5ltybaSWjKIwjYJfIXuoyzE7OL63856MC/Os8PcLfY7vYY2LB cvKH1qOcs+an86DWX17+dkcKD/YLrpzwvRMur5+kTgVfXcC0TAl39N4YtaCKM/3ugAaVS1Mw MrbyGnGqVMqlCpjnpYREzapSk8XxbO2kYRsZQd8J9ei98OSqgPf8xM7NCULd/xaZLJUydql1 JdSREId2C15jut21aQARAQABwsF2BBgBCgAgFiEEO5YHwQksFRkI+NPdJGddkSfKF+wFAmcr pvICGwwACgkQJGddkSfKF+xuuxAA4F9iQc61wvAOAidktv4Rztn4QKy8TAyGN3M8zYf/A5Zx VcGgX4J4MhRUoPQNrzmVlrrtE2KILHxQZx5eQyPgixPXri42oG5ePEXZoLU5GFRYSPjjTYmP ypyTPN7uoWLfw4TxJqWCGRLsjnkwvyN3R4161Dty4Uhzqp1IkNhl3ifTDYEvbnmHaNvlvvna 7+9jjEBDEFYDMuO/CA8UtoVQXjy5gtOhZZkEsptfwQYc+E9U99yxGofDul7xH41VdXGpIhUj 4wjd3IbgaCiHxxj/M9eM99ybu5asvHyMo3EFPkyWxZsBlUN/riFXGspG4sT0cwOUhG2ZnExv XXhOGKs/y3VGhjZeCDWZ+0ZQHPCL3HUebLxW49wwLxvXU6sLNfYnTJxdqn58Aq4sBXW5Un0Q vfbd9VFV/bKFfvUscYk2UKPi9vgn1hY38IfmsnoS8b0uwDq75IBvup9pYFyNyPf5SutxhFfP JDjakbdjBoYDWVoaPbp5KAQ2VQRiR54lir/inyqGX+dwzPX/F4OHfB5RTiAFLJliCxniKFsM d8eHe88jWjm6/ilx4IlLl9/MdVUGjLpBi18X7ejLz3U2quYD8DBAGzCjy49wJ4Di4qQjblb2 pTXoEyM2L6E604NbDu0VDvHg7EXh1WwmijEu28c/hEB6DwtzslLpBSsJV0s1/jE= In-Reply-To: <20251124125249.GDaSRVIapy2dmis28p@fat_crate.local> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11/24/25 14:52, Borislav Petkov wrote: > On Mon, Nov 24, 2025 at 08:03:02PM +0800, Kuan-Wei Chiu wrote: >> On Mon, Nov 24, 2025 at 12:05:26PM +0100, Borislav Petkov wrote: >>> On Mon, Nov 24, 2025 at 04:57:51PM +0800, Kuan-Wei Chiu wrote: >>>>> Both LLVM/GCC support a __builtin_parity function which is functionally >>>>> equivalent to the custom bitwise_xor_bits() one. Let's simplify the code by >>>>> relying on the built-in. No functional changes. >>>> >>>> IIRC in some cases, >>> >>> Which are those cases? >>> >>> Do you have a trigger scenario? >>> >> I did a quick search, and I believe it was this kernel test robot >> report [1] that reminded me of this compiler behavior. >> >> [1]: https://lore.kernel.org/oe-kbuild-all/202501312159.l6jNRaYy-lkp@intel.com/ > > Interesting, thanks for the pointer. > > @Nik, just use hweight16() but pls do check what asm the compiler generates > before and after so that at least there's some palpable improvement or gcc is > smart enough to replace the unrolled XORing with something slick. > > Also put in the commit message why we're not using the builtin parity thing > and quote the link above. > > Thx. > So the custom function one results in the following asm being inlined i.e 4 total copies: # drivers/ras/amd/atl/umc.c:59: tmp ^= (val >> i) & 0x1; xorl %ecx, %ecx # ivtmp.157 # drivers/ras/amd/atl/umc.c:55: u16 tmp = 0; xorl %esi, %esi # tmp # drivers/ras/amd/atl/umc.c:253: temp = bitwise_xor_bits(col & addr_hash.bank[i].col_xor); andl %r13d, %edx # col, tmp249 # drivers/ras/amd/atl/umc.c:59: tmp ^= (val >> i) & 0x1; movzwl %dx, %edx # tmp249, _387 .L3: movl %edx, %eax # _387, tmp250 sarl %cl, %eax # ivtmp.157, tmp250 # drivers/ras/amd/atl/umc.c:58: for (i = 0; i < 16; i++) addl $1, %ecx #, ivtmp.157 # drivers/ras/amd/atl/umc.c:59: tmp ^= (val >> i) & 0x1; andl $1, %eax #, tmp251 # drivers/ras/amd/atl/umc.c:59: tmp ^= (val >> i) & 0x1; xorl %eax, %esi # tmp251, tmp # drivers/ras/amd/atl/umc.c:58: for (i = 0; i < 16; i++) cmpl $16, %ecx #, ivtmp.157 jne .L3 #, Whilst with hweight, if the cpu has popcnt it will be 2 instructions - popcnt and an andl: # drivers/ras/amd/atl/umc.c:243: temp ^= hweight16(row & addr_hash.bank[i].row_xor) & 1; andl %ebp, %edi # _321, tmp221 # ./arch/x86/include/asm/arch_hweight.h:19: asm_inline (ALTERNATIVE("call __sw_hweight32", #APP # 19 "./arch/x86/include/asm/arch_hweight.h" 1 # ALT: oldinstr 771: call __sw_hweight32 772: # ALT: padding .skip -(((775f-774f)-(772b-771b)) > 0) * ((775f-774f)-(772b-771b)),0x90 773: .pushsection .altinstructions,"a" .long 771b - . .long 774f - . .4byte ( 4*32+23) .byte 773b-771b .byte 775f-774f .popsection .pushsection .altinstr_replacement, "ax" # ALT: replacement 774: popcntl %edi, %eax # tmp221, res 775: .popsection # 0 "" 2 # drivers/ras/amd/atl/umc.c:243: temp ^= hweight16(row & addr_hash.bank[i].row_xor) & 1; #NO_APP xorl %eax, %edx # res, tmp222 andl $1, %edx #, temp So yeah, much better IMHO, unless there is some hidden latency in the popcnt...