From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 3FA061519BA; Mon, 9 Jun 2025 22:59:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749509989; cv=none; b=ZOPa2M06NoGk7s06kB8cZCvRq33J7SjKW4zuJ8o0BhVWQAx2ehJpfhfxd+6A7gokJvoOafdP9IWgYgDU+3gpXv55Ns99+nIfuKq/j1vhdOVJ2zRyVNuaKhuvrEZD6CmzuSZx6M4z7teVKtI7BuOfTtRTQu+z6qIMpeOw9vCWNEc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749509989; c=relaxed/simple; bh=U4vhsJXM46e+0Mfc4PBs3bseTa3h9UosBlHtozqeIf8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=B7JCme5vD0DNL+6n75FdM5qPiHxCgzKtZ/4pNt3j8hjwHqc1d2wsL3PoGSMdDsvfU0BFtxyQErKVNE9RQC2jNUkf54etvqASNiM+/Upd7+7TlZvYS57oDemdCNiCQLmE5vdGx7pVbf9ZV1qhUVIYc+YmYxu7Vc8u9U1z59GgC3s= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=bZ2q/LWi; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="bZ2q/LWi" 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> Precedence: bulk X-Mailing-List: linux-crypto@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 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