Message-ID: <20250123082215.GC183612@sol.localdomain>
Date: Thu, 23 Jan 2025 00:22:15 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ard Biesheuvel <ardb@...nel.org>, Chao Yu <chao@...nel.org>,
	"Darrick J. Wong" <djwong@...nel.org>,
	Kent Overstreet <kent.overstreet@...ux.dev>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Theodore Ts'o <tytso@....edu>,
	Vinicius Peixoto <vpeixoto@...amp.dev>,
	WangYuli <wangyuli@...ontech.com>
Subject: Re: [GIT PULL] CRC updates for 6.14

On Thu, Jan 23, 2025 at 09:16:21AM +0100, Geert Uytterhoeven wrote:
> Hi Eric,
> 
> On Thu, Jan 23, 2025 at 6:16 AM Eric Biggers <ebiggers@...nel.org> wrote:
> > On Wed, Jan 22, 2025 at 08:13:07PM -0800, Linus Torvalds wrote:
> > > On Sun, 19 Jan 2025 at 14:51, Eric Biggers <ebiggers@...nel.org> wrote:
> > > >
> > > > - Reorganize the architecture-optimized CRC32 and CRC-T10DIF code to be
> > > >   directly accessible via the library API, instead of requiring the
> > > >   crypto API.  This is much simpler and more efficient.
> > >
> > > I'm not a fan of the crazy crypto interfaces for simple hashes that
> > > only complicate and slow things down, so I'm all in favor of this and
> > > have pulled it.
> > >
> > > HOWEVER.
> > >
> > > I'm also very much not a fan of asking users pointless questions.
> > >
> > > What does this patch-set ask users idiotic questions like
> > >
> > >   CRC-T10DIF implementation
> > >   > 1. Architecture-optimized (CRC_T10DIF_IMPL_ARCH) (NEW)
> > >     2. Generic implementation (CRC_T10DIF_IMPL_GENERIC) (NEW)
> > >
> > > and
> > >
> > >   CRC32 implementation
> > >   > 1. Arch-optimized, with fallback to slice-by-8
> > > (CRC32_IMPL_ARCH_PLUS_SLICEBY8) (NEW)
> > >     2. Arch-optimized, with fallback to slice-by-1
> > > (CRC32_IMPL_ARCH_PLUS_SLICEBY1) (NEW)
> > >     3. Slice by 8 bytes (CRC32_IMPL_SLICEBY8) (NEW)
> > >     4. Slice by 4 bytes (CRC32_IMPL_SLICEBY4) (NEW)
> > >     5. Slice by 1 byte (Sarwate's algorithm) (CRC32_IMPL_SLICEBY1) (NEW)
> > >     6. Classic Algorithm (one bit at a time) (CRC32_IMPL_BIT) (NEW)
> > >
> > > because *nobody* wants to see that completely pointless noise.
> > >
> > > Pick the best one. Don't ask the user to pick the best one.
> > >
> > > If you have some really strong argument for why users need to be able
> > > to override the sane choice, make the question it at *least* depend on
> > > EXPERT.
> > >
> > > And honestly, I don't see how there could possibly ever be any point.
> > > If there is an arch-optimized version, just use it.
> > >
> > > And if the "optimized" version is crap and worse than some generic
> > > one, it just needs to be removed.
> > >
> > > None of this "make the user make the choice because kernel developers
> > > can't deal with the responsibility of just saying what is best".
> >
> > Yes, I agree, and the kconfig options are already on my list of things to clean
> > up.  Thanks for giving your thoughts on how to do it.  To be clarify, this
> > initial set of changes removed the existing arch-specific CRC32 and CRC-T10DIF
> > options (on x86 that was CRYPTO_CRC32C_INTEL, CRYPTO_CRC32_PCLMUL, and
> > CRYPTO_CRCT10DIF_PCLMUL) and added the equivalent functionality to two choices
> > in lib, one of which already existed.  So for now the changes to the options
> > were just meant to consolidate them, not add to or remove from them per se.
> >
> > I do think that to support kernel size minimization efforts we should continue
> > to allow omitting the arch-specific CRC code.  One of the CRC options, usually
> > CONFIG_CRC32, gets built into almost every kernel.  Some options already group
> > together multiple CRC variants (e.g. there are three different CRC32's), and
> > each can need multiple implementations targeting different instruction set
> > extensions (e.g. both PCLMULQDQ and VPCLMULQDQ on x86).  So it does add up.
> >
> > But it makes sense to make the code be included by default, and make the choice
> > to omit it be conditional on CONFIG_EXPERT.
> >
> > I'm also thinking of just doing a single option that affects all enabled CRC
> > variants, e.g. CRC_OPTIMIZATIONS instead of both CRC32_OPTIMIZATIONS and
> > CRC_T10DIF_OPTIMIZATIONS.  Let me know if you think that would be reasonable.
> >
> > As you probably noticed, the other problem is that CRC32 has 4 generic
> > implementations: bit-by-bit, and slice by 1, 4, or 8 bytes.
> >
> > Bit-by-bit is useless.  Slice by 4 and slice by 8 are too similar to have both.
> >
> > It's not straightforward to choose between slice by 1 and slice by 4/8, though.
> > When benchmarking slice-by-n, a higher n will always be faster in
> > microbenchmarks (up to about n=16), but the required table size also increases
> > accordingly.  E.g., a slice-by-1 CRC32 uses a 1024-byte table, while slice-by-8
> > uses a 8192-byte table.  This table is accessed randomly, which is really bad on
> > the dcache, and can be really bad for performance in real world scenarios where
> > the system is bottlenecked on memory.
> >
> > I'm tentatively planning to just say that slice-by-4 is a good enough compromise
> > and have that be the only generic CRC32 implementation.
> 
> So I guess I want slice-by-1 on m68k. Or
> 
>     default CRC32_IMPL_SLICEBY1 if CONFIG_CC_OPTIMIZE_FOR_SIZE
> 
> so I don't have to touch all defconfigs? ;-)

As I mentioned in my next reply I'm actually leaning towards slice-by-1 only
now.

> BTW, shouldn't all existing defconfigs that enable
> CONFIG_CRC32_SLICEBY[48], CONFIG_CRC32_SARWATE, or CRC32_BIT be updated,
> as the logic has changed (these symbols are now enabled based on
> CRC32_IMPL*)?

Yes, though I doubt that anyone who was selecting a specific generic CRC32
implementation really put much thought into it.  And if we standardize on one
implementation then the choice will go away and it won't matter anyway.

- Eric

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