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 67A5A41A4EF; Thu, 1 Oct 2026 09:25:45 +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=1790846747; cv=none; b=YOeOlpF5ebj3R6O8CJnachJ1eaa1rJi28ANNaoa4wAVjrpN7F4efRUFoVA6gEFDijxLWaIXQEsS8XsW27TQbltSjaaEBhAIaqtUqmWlHPD/YWr3cHLcdJrcn4saCb2+7hq2MWFUUJkdfW8IXmWkp/VjmNt4OBq13+jKF+iEskH0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790846747; c=relaxed/simple; bh=HUYkGqcZQJs3k4zIDyWA4OZP7ikrl14JcASJcrAM4ZA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=p0MOPm0Vajpjls8sBN09shnGXPDz/YWylOpIXIaiZ7rq0h7MPJNXzSTUrj2Z2w3pg+J9tSNHVj28OoBNaC0XYEh1SYAgzuRUg8k75JIYi8Nqm7yYk1m0P/CcDRwrMrWAt9JHLxtqfT/W5awCgWHtueL8OnoLCG4D+KIWn9bc5Mg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b=WWTEvHs+; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b="WWTEvHs+" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EDC6E1F000FF; Thu, 1 Oct 2026 09:25:41 +0000 (UTC) Authentication-Results: smtp.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=zx2c4.com header.i=@zx2c4.com header.a=rsa-sha256 header.s=20210105 header.b=WWTEvHs+ DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zx2c4.com; s=20210105; t=1790846740; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=dRiq/gc/1/v3q1z4vqQ1Z479kjp0uI+oeZqHFxoYK3k=; b=WWTEvHs+Glx9cwBY3B1J4eFsMVayAa8DZTePcEN0LwV8619xjbZb+A3Te+DbQ8bzZJ95Rl nakLPPxk3P6NpEgkw8BAvLRde47zWZGiGtwlUURKwXZEieW6g5dYOrgW3q1U+6OPmcgwQQ /6QP7vXzfWvWvdrBScZiV3GjkkHM4XE= Received: by mail.zx2c4.com (OpenSMTPD) with ESMTPSA id d847d3a4 (TLSv1.3:TLS_AES_256_GCM_SHA384:256:NO); Thu, 1 Oct 2026 09:25:39 +0000 (UTC) Date: Thu, 1 Oct 2026 11:25:35 +0200 From: "Jason A. Donenfeld" To: "Christophe Leroy (CS GROUP)" Cc: Nathan Chancellor , 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 Subject: Re: [PATCH v2] random: vDSO: Avoid call to memset() when zeroing reserved in __cvdso_getrandom_data() Message-ID: References: <9d1c338b-368f-4ea2-a12e-c28d49adff5e@kernel.org> <20260930133813.GA3142230@ax162> <20260930151337.GD3142230@ax162> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Thu, Oct 01, 2026 at 06:41:20AM +0200, Christophe Leroy (CS GROUP) wrote: > 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. Darn. Yea. The naive memset kills optimizations. Okay, new strategy: - on clang, pass `-mllvm -max-store-memset=4294967295` - on gcc, pass `-finline-stringops=memset` And keep the same code. (Or, better, see if those options generate good code with b7bad082e113640fc81200ff869e5c2d7a9c29a2 reverted; I would prefer that simpler initializer.) Nathan, do these work? Jason