From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DE04736B939; Thu, 1 Oct 2026 04:41:31 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790829693; cv=none; b=BrQpEAiCJ87BvpClWee0kBYLIdPFRCZ/fopmfg6DkrcQ8LhfMrJwyvNutOl9xxOix5sNjPE5QptMQbk7tKiKK2n1d5KXWw/IJvRnr30mO7RgZna9ZML3phFZiB5B3Nwxx+3SmY6xSjYyGrRBuss8BouRxpp5qFLHhp9G9E+IdtI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790829693; c=relaxed/simple; bh=LJabXzDr8Pqmi2JYmBKjyWjaEcW0q9oUv6Q7p35WK6U=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=elF3HaCwhLb7wVhlIMarygE+Ubslhmql/+6e36/eVwgfKMfgSOVL6mbO8WTXxdRDo3laYlrJqE0aUaPohFfEGFT/2WaB+Fo657Jbg8it0F43QPzKb52B8WdEHkj8yhsPLQFzZjaLf6RA62QSkWyx3g96emitl9GeDKx5csJVCIo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Bq0nZkKW; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Bq0nZkKW" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B96EF1F000FF; Thu, 1 Oct 2026 04:41:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790829691; bh=tVU5xB7+vpzEDdoS9i3GNs5XYI1dWTYZUYdxVIutOtM=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Bq0nZkKWMqWIxU4tIv4zapeOdCpnHz5b/zR/btuwhOI6yqWhkNa16ZLAdV0XAonaE LT/7g8b0EEn8q6WkaYSXcge2H8Yc3IFapoddC+KtKRElzzzS2tCykxpi6k5aZA9kay 7Bx2fP9pScRcEmAWGN5VHfJ7AF5BwdkPUQT7ytyICmEnBPU0xblgkviAuLC9gCD0Mc 1wW64Qj4Bi6OFUgghHfnyrW6ZOr1ms95vNMMwVa/6E/AhGbiTu69p6opZ+oOc4sLrS iIWKzWRMviz1dpodiBtpGvwGIBrsrPpmtw65JP7qKlykYyEAPSVgsAOV08268AC+o8 rWTxHJHwYW1TQ== Message-ID: Date: Thu, 1 Oct 2026 06:41:20 +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] random: vDSO: Avoid call to memset() when zeroing reserved in __cvdso_getrandom_data() To: "Jason A. Donenfeld" , Nathan Chancellor Cc: Nick Desaulniers , Andy Lutomirski , Thomas Gleixner , Theodore Ts'o , Vincenzo Frascino , Bill Wendling , Justin Stitt , Catalin Marinas , Will Deacon , Mark Rutland , Huacai Chen , WANG Xuerui , Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Christian Borntraeger , Sven Schnelle , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , x86@kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, loongarch@lists.linux.dev, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, llvm@lists.linux.dev References: <9d1c338b-368f-4ea2-a12e-c28d49adff5e@kernel.org> <20260930133813.GA3142230@ax162> <20260930151337.GD3142230@ax162> Content-Language: fr-FR From: "Christophe Leroy (CS GROUP)" In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Hi, Le 30/09/2026 à 17:16, Jason A. Donenfeld a écrit : > On Wed, Sep 30, 2026 at 5:13 PM Nathan Chancellor wrote: >> >> On Wed, Sep 30, 2026 at 04:44:29PM +0200, Jason A. Donenfeld wrote: >>> Nathan, would this be okay with you? >>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.zx2c4.com%2Flinux-rng%2Fcommit%2F%3Fid%3Dd216701724b7d8209ff42150658ad5c712bdb503&data=05%7C02%7Cchristophe.leroy2%40cs-soprasteria.com%7C8f69935bffb344bf0ddc08df1f05e358%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639263782324080837%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=BA7Fyr0%2FynggvBnR%2FpiCvNCQHtNBagqvSb0i11AvoLU%3D&reserved=0 >> >> Can you stick >> >> Cc: stable@vger.kernel.org # v6.12+ >> Closes: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FClangBuiltLinux%2Flinux%2Fissues%2F2183&data=05%7C02%7Cchristophe.leroy2%40cs-soprasteria.com%7C8f69935bffb344bf0ddc08df1f05e358%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639263782324110679%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=6C8%2FAEQoh%2B8pfgfYQkL7qf5Ue%2F5sAGtfqRptzsSG0CM%3D&reserved=0 >> >> on that? Otherwise, looks good to me, that's basically what I had for my >> v3 locally. > > Sure, done. Also removed the now-unused array_size.h include. I'm still very sceptic with this patch. You are degrading the behaviour with GCC for a problem with CLANG. Why ? Before the patch, with both GCC 13 and GCC 16 on powerpc32 I get a pretty standard optimised loop that clears words 4 by 4 (with auto increment of pointer) which is the most optimal on powerpc: 3f0: 39 00 00 0c li r8,12 3f4: 35 08 ff fc addic. r8,r8,-4 3f8: 91 49 00 04 stw r10,4(r9) 3fc: 91 49 00 08 stw r10,8(r9) 400: 91 49 00 0c stw r10,12(r9) 404: 95 49 00 10 stwu r10,16(r9) 408: 40 82 ff ec bne 3f4 <__c_kernel_getrandom+0x3f4> With the patch, With GCC 13 I get a very suboptimal loop copying bytes one by one 3d8: 39 40 00 34 li r10,52 ... 3e4: 39 20 00 00 li r9,0 3e8: 7d 49 03 a6 mtctr r10 3ec: 9d 3e 00 01 stbu r9,1(r30) 3f0: 42 00 ff fc bdnz 3ec <__c_kernel_getrandom+0x3ec> With GCC 16 I get something a bit better but not as good as before, it is a loop clearing words only one by one and incrementing pointer with an additional insn instead of using auto-increment instruction stwu. 3e0: 39 40 00 0d li r10,13 ... 3f0: 7d 49 03 a6 mtctr r10 3f4: 91 3f 00 00 stw r9,0(r31) 3f8: 3b ff 00 04 addi r31,r31,4 3fc: 42 00 ff f8 bdnz 3f4 <__c_kernel_getrandom+0x3f4> Please restrict the patch to clang builds. Christophe