linux-kernel - Re: [PATCH v4 08/21] lib/crc: Switch ARM and arm64 to 'ksimd' scoped guard API

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Message-ID: <CAMj1kXHMa_Vj3DsuoAR-rvWW12Bsnz10w+BAze6mtngqpABZPw@mail.gmail.com>
Date: Fri, 31 Oct 2025 14:52:36 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Jonathan Cameron <jonathan.cameron@...wei.com>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org, 
	herbert@...dor.apana.org.au, ebiggers@...nel.org
Subject: Re: [PATCH v4 08/21] lib/crc: Switch ARM and arm64 to 'ksimd' scoped
 guard API

On Fri, 31 Oct 2025 at 14:49, Jonathan Cameron
<jonathan.cameron@...wei.com> wrote:
>
> On Fri, 31 Oct 2025 11:39:07 +0100
> Ard Biesheuvel <ardb+git@...gle.com> wrote:
>
> > From: Ard Biesheuvel <ardb@...nel.org>
> >
> > Before modifying the prototypes of kernel_neon_begin() and
> > kernel_neon_end() to accommodate kernel mode FP/SIMD state buffers
> > allocated on the stack, move arm64 to the new 'ksimd' scoped guard API,
> > which encapsulates the calls to those functions.
> >
> > For symmetry, do the same for 32-bit ARM too.
> >
> > Reviewed-by: Eric Biggers <ebiggers@...nel.org>
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > ---
> >  lib/crc/arm/crc-t10dif.h   | 16 +++++-----------
> >  lib/crc/arm/crc32.h        | 11 ++++-------
> >  lib/crc/arm64/crc-t10dif.h | 16 +++++-----------
> >  lib/crc/arm64/crc32.h      | 16 ++++++----------
> >  4 files changed, 20 insertions(+), 39 deletions(-)
> >
> > diff --git a/lib/crc/arm/crc-t10dif.h b/lib/crc/arm/crc-t10dif.h
> > index 63441de5e3f1..aaeeab0defb5 100644
> > --- a/lib/crc/arm/crc-t10dif.h
> > +++ b/lib/crc/arm/crc-t10dif.h
>
> >  static __ro_after_init DEFINE_STATIC_KEY_FALSE(have_neon);
> > @@ -20,21 +19,16 @@ asmlinkage void crc_t10dif_pmull8(u16 init_crc, const u8 *buf, size_t len,
> >  static inline u16 crc_t10dif_arch(u16 crc, const u8 *data, size_t length)
> >  {
> >       if (length >= CRC_T10DIF_PMULL_CHUNK_SIZE) {
> > -             if (static_branch_likely(&have_pmull)) {
> > -                     if (likely(may_use_simd())) {
> > -                             kernel_neon_begin();
> > -                             crc = crc_t10dif_pmull64(crc, data, length);
> > -                             kernel_neon_end();
> > -                             return crc;
> > -                     }
> > +             if (static_branch_likely(&have_pmull) && likely(may_use_simd())) {
> > +                     scoped_ksimd()
> > +                             return crc_t10dif_pmull64(crc, data, length);
>
> >               } else if (length > CRC_T10DIF_PMULL_CHUNK_SIZE &&
> >                          static_branch_likely(&have_neon) &&
> >                          likely(may_use_simd())) {
>
> I briefly thought this was a functional change but it's not because
> of may_use_simd() being something that isn't going to change between
> the two evaluations.
>
> Would it hurt at all to pull that up to be
>         if (length >= CRC_T10DIF_PMULL_CHUNK_SIZE && likely(may_use_simd())) {
>                 if (static_branch_likely(&have_pmull)) {
>                         scoped_ksimd()
>                                 return crc_t10dif_pmull64(crc, data, length);
>
>                 } else if (length > CRC_T10DIF_PMULL_CHUNK_SIZE &&
>                            static_branch_likely(&have_neon)) {
>                 ...
>
> ?
>

Yeah that would be a reasonable cleanup, I guess.