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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 9DF8FCA5FB3 for ; Thu, 1 Oct 2026 09:25:54 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To: Content-Transfer-Encoding:Content-Type:MIME-Version:References:Message-ID: Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=dRiq/gc/1/v3q1z4vqQ1Z479kjp0uI+oeZqHFxoYK3k=; b=syrVIzFtne9grumrhnP3m+O4Dq Tt6Psadyq/pyInbPuotQsySpUVxcbLsemwgBKeQJZ1kd0uchZXlg3/pTwYAObQc4AIrpUpCDaGS6X 3x00l1i+PBNFH+35CtXVsH/58LHcQPE9HgmdLRU998RH7owqw+TfmloJNeP0AS1zeVn3L62hMlsBL +LKQKCaul4tEKaXbUyUtaE5mKU0cagGXSuCscpbmnnH0IJIBUpuEdMwRtXsiHJnGFo3JMjyrXPORg JoHo8eFuI6502ITQDcpuG1lSnQx2uP9xiZF/+zmFOPnB9jzXLi4847Rlla8jyaPZXOtgDRfbSEs2x xSOBT49A==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1xCD2x-00000008M52-2Nym; Thu, 01 Oct 2026 09:25:47 +0000 Received: from tor.source.kernel.org ([172.105.4.254]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1xCD2w-00000008M4t-1KWG; Thu, 01 Oct 2026 09:25:46 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 6B4876022B; Thu, 1 Oct 2026 09:25:45 +0000 (UTC) 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> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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