Re: [GIT PULL] CRC updates for 6.14

Eric Biggers
2025-01-23T08:26:18Z

On Thu, Jan 23, 2025 at 09:19:53AM +0100, Geert Uytterhoeven wrote:
> Hi Eric,
>
> On Thu, Jan 23, 2025 at 9:16 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > On Thu, Jan 23, 2025 at 6:16 AM Eric Biggers <ebiggers@kernel.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@kernel.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? ;-)
> >
> > 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*)?
>
> Oh, I just realized m68k used CONFIG_CRC32_SLICEBY8=y before, as that
> was the default. So all these questions are not new, just churn because
> of the changed logic?
>
> Gr{oetje,eeting}s,

Yes, there was already a choice for CRC32 implementation before, with SLICEBY8
still being the default. It just came up again because I changed the options in
the choice by adding the arch-specific options (which previously were in a
different location) to it.

- Eric