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 8C430C5B552 for ; Mon, 9 Jun 2025 23:02:15 +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=F2w5rX95eeScpv5mqK21JWv/UhtBCWE6Zq/RxFSbaLM=; b=rlhFmHTzs7Tht8fUMmSpnHzyl+ DYr4zj+DSQy5cFYdL/VkY/2JR+oxreLvJoqmTYgvYIEiTkIJ1E9+b1+ZQX1BS9jwB0obyF15mh/Y4 jxENYB+Eqbifg8Y3NkcXZxPJmZFw8FOZ+Pj9I8XL9teeI2Do79VRjOp/wQSMgayVw31xiXB9BH/WQ 1BpXW2aTOYdSjrY6Q1Z7W8uXX9QQ+Dp79BLHXPfzlQRs5NtBx2r++fJFGBLVnmd3zJFch3YG3kfx2 MgVKsY/k+blgQGIqWwWAG+a79k+nX5XaWAjPtFWJXQUPSZ86/9hYOrnumBMYgoPv07fuMVJtudrjD 8t08KpzQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1uOlVD-00000005PY8-1j4G; Mon, 09 Jun 2025 23:02:03 +0000 Received: from sea.source.kernel.org ([2600:3c0a:e001:78e:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1uOlT3-00000005POT-20nW; Mon, 09 Jun 2025 22:59:50 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id B726D4A946; Mon, 9 Jun 2025 22:59:48 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 21A46C4CEEB; Mon, 9 Jun 2025 22:59:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1749509988; bh=U4vhsJXM46e+0Mfc4PBs3bseTa3h9UosBlHtozqeIf8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=bZ2q/LWijEpaEq00igAvU15ZFA2btFmPEoPv0oTqawn+MjrxKEc4/OqXy2N38kxpV AFvy2lrAzn2djDaUY1swfzB9EZ627MitJD6/avaXEMt58zpc+17thB5+o+L/NCFmXx Bbahqr8beEaE9JLR92svUSP/oo9HcH3K/MV2jUx5drdibhmczFNStuOfZD9lCWMysz P11+Cs72P7BkkpWfSW0Wc/UiWNcR1xk5ksNxTizKDn6BSr0mEtJF/ezlntceoWJxAz yys2b1O4D36HQh8H0QW/vQ0doPIIwaleXhHsMUSvkGTKjPZAQluFZIrFMruyPgxt0i IRxGum7RmzoYw== Date: Mon, 9 Jun 2025 15:59:26 -0700 From: Eric Biggers To: Julian Calaby Cc: linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org, linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, sparclinux@vger.kernel.org, x86@kernel.org, linux-arch@vger.kernel.org, Ard Biesheuvel , "Jason A . Donenfeld" , Linus Torvalds Subject: Re: [PATCH v2 00/12] lib/crc: improve how arch-optimized code is integrated Message-ID: <20250609225926.GE1255@sol> References: <20250607200454.73587-1-ebiggers@kernel.org> <20250609194845.GC1255@sol> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20250609_155949_558387_DE4D53DF X-CRM114-Status: GOOD ( 41.43 ) 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 Tue, Jun 10, 2025 at 08:36:39AM +1000, Julian Calaby wrote: > Hi Eric, > > On Tue, Jun 10, 2025 at 5:49 AM Eric Biggers wrote: > > > > On Mon, Jun 09, 2025 at 06:15:24PM +1000, Julian Calaby wrote: > > > Hi Eric, > > > > > > On Sun, Jun 8, 2025 at 6:07 AM Eric Biggers wrote: > > > > > > > > This series is also available at: > > > > > > > > git fetch https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git lib-crc-arch-v2 > > > > > > > > This series improves how lib/crc supports arch-optimized code. First, > > > > instead of the arch-optimized CRC code being in arch/$(SRCARCH)/lib/, it > > > > will now be in lib/crc/$(SRCARCH)/. Second, the API functions (e.g. > > > > crc32c()), arch-optimized functions (e.g. crc32c_arch()), and generic > > > > functions (e.g. crc32c_base()) will now be part of a single module for > > > > each CRC type, allowing better inlining and dead code elimination. The > > > > second change is made possible by the first. > > > > > > > > As an example, consider CONFIG_CRC32=m on x86. We'll now have just > > > > crc32.ko instead of both crc32-x86.ko and crc32.ko. The two modules > > > > were already coupled together and always both got loaded together via > > > > direct symbol dependency, so the separation provided no benefit. > > > > > > > > Note: later I'd like to apply the same design to lib/crypto/ too, where > > > > often the API functions are out-of-line so this will work even better. > > > > In those cases, for each algorithm we currently have 3 modules all > > > > coupled together, e.g. libsha256.ko, libsha256-generic.ko, and > > > > sha256-x86.ko. We should have just one, inline things properly, and > > > > rely on the compiler's dead code elimination to decide the inclusion of > > > > the generic code instead of manually setting it via kconfig. > > > > > > > > Having arch-specific code outside arch/ was somewhat controversial when > > > > Zinc proposed it back in 2018. But I don't think the concerns are > > > > warranted. It's better from a technical perspective, as it enables the > > > > improvements mentioned above. This model is already successfully used > > > > in other places in the kernel such as lib/raid6/. The community of each > > > > architecture still remains free to work on the code, even if it's not in > > > > arch/. At the time there was also a desire to put the library code in > > > > the same files as the old-school crypto API, but that was a mistake; now > > > > that the library is separate, that's no longer a constraint either. > > > > > > Quick question, and apologies if this has been covered elsewhere. > > > > > > Why not just use choice blocks in Kconfig to choose the compiled-in > > > crc32 variant instead of this somewhat indirect scheme? > > > > > > This would keep the dependencies grouped by arch and provide a single place to > > > choose whether the generic or arch-specific method is used. > > > > It's not clear exactly what you're suggesting, but it sounds like you're > > complaining about this: > > > > config CRC32_ARCH > > bool > > depends on CRC32 && CRC_OPTIMIZATIONS > > default y if ARM && KERNEL_MODE_NEON > > default y if ARM64 > > default y if LOONGARCH > > default y if MIPS && CPU_MIPSR6 > > default y if PPC64 && ALTIVEC > > default y if RISCV && RISCV_ISA_ZBC > > default y if S390 > > default y if SPARC64 > > default y if X86 > > I was suggesting something roughly like: > > choice > prompt "CRC32 Variant" > depends on CRC32 && CRC_OPTIMIZATIONS > > config CRC32_ARCH_ARM_NEON > bool "ARM NEON" > default y > depends ARM && KERNEL_MODE_NEON > > ... > > config CRC32_GENERIC > bool "Generic" > > endchoice > > > This patchset strikes a balance where the vast majority of the arch-specific CRC > > code is isolated in lib/crc/$(SRCARCH), and the exceptions are just > > lib/crc/Makefile and lib/crc/Kconfig. I think these exceptions make sense, > > given that we're building a single module per CRC variant. We'd have to go > > through some hoops to isolate the arch-specific Kconfig and Makefile snippets > > into per-arch files, which don't seem worth it here IMO. > > I was only really concerned with the Kconfig structure, I was > expecting Kbuild to look roughly like this: (filenames are wrong) > > crc32-y += crc32-base.o > crc32-$(CRC32_ARCH_ARM_NEON) += arch/arm/crc32-neon.o > ... > crc32-$(CRC32_GENERIC) += crc32-generic.o > > but yeah, your proposal here has grown on me now that I think about it > and the only real "benefit" mine has is that architectures can display > choices for variants that have Kconfig-visible requirements, which > probably isn't that many so it wouldn't be useful in practice. > > Thanks for answering my question, The CRC32 implementation did used to be user-selectable, but that was already removed in v6.14 (except for the coarse-grained knob CONFIG_CRC_OPTIMIZATIONS that remains and can be disabled only when CONFIG_EXPERT=y) since the vast majority of users simply want the optimized CRC32 code enabled. The fact that it wasn't just enabled by default was a longstanding bug. - Eric