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Date: Mon, 29 Jun 2026 14:21:52 +0200
From: Christoph Hellwig <hch@lst.de>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Christoph Hellwig <hch@lst.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Ard Biesheuvel <ardb@kernel.org>,
	Huacai Chen <chenhuacai@kernel.org>,
	WANG Xuerui <kernel@xen0n.name>,
	Madhavan Srinivasan <maddy@linux.ibm.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Nicholas Piggin <npiggin@gmail.com>,
	"Christophe Leroy (CS GROUP)" <chleroy@kernel.org>,
	Paul Walmsley <pjw@kernel.org>, Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>, Alexandre Ghiti <alex@ghiti.fr>,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	Sven Schnelle <svens@linux.ibm.com>,
	Thomas Gleixner <tglx@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
	Herbert Xu <herbert@gondor.apana.org.au>,
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	David Sterba <dsterba@suse.com>, Arnd Bergmann <arnd@arndb.de>,
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	linux-btrfs@vger.kernel.org, linux-arch@vger.kernel.org,
	linux-raid@vger.kernel.org
Subject: Re: cleanup the RAID6 P/Q library v3
Message-ID: <20260629122152.GA21344@lst.de>
References: <20260518051804.462141-1-hch@lst.de> <20260518141205.c100f76eec5f58e78bbbf7af@linux-foundation.org> <20260519082432.GA14956@lst.de> <7d8efc7c-65e3-49f8-9381-910de7affef2@zytor.com>
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On Fri, Jun 26, 2026 at 05:52:18PM -0700, H. Peter Anvin wrote:
> On 2026-05-19 01:24, Christoph Hellwig wrote:
> > 
> > raid6: rework registration of optimized algorithms
> > 
> >  - avx2 instead of avx512 is probably the right thing for no
> >    benchmarking, but if it was intentional (it wasn't), that should
> >    be document.  So I'll just switch back to the previous version to
> >    keep the state of the art
> 
> It is unlikely to be the right thing *going forward*, though.
> 
> The very unfortunate performance inversion is likely model-specific. It is one
> of those things where you largely would have to have a list of quirks :(

Note that the default is still to benchmark the implementations.
This is just about the case where the user explicitly disabled that
benchmark-based selection.