Re: [PATCH 3/3] crypto: x86 - Opt out of initializing MXCSR and 387 state

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Nov 04, 2026 at 04:55:23PM -0800, Eric Biggers wrote:
> The crypto code using "kernel-mode FPU" is integer code that does not
> use any MMX or x87 instructions.  To the extent that float-domain
> instructions are used at all, they are only moves and shuffles, neither
> of which raises SIMD floating-point exceptions or depends on MXCSR.
> 
> Therefore, there's no need to initialize MXCSR or the 387 FPU state.
> 
> Opt out of this initialization by using kernel_fpu_begin_mask(0) instead
> of kernel_fpu_begin().  This eliminates an 'ldmxcsr' instruction in
> every kernel-mode FPU section, reducing overhead, which matters most for
> short input lengths.  On 32-bit kernels it also eliminates 'fninit'.
> 
> A precedent for this optimization can be found in
> commit a58db7ad80e8 ("netfilter: nft_set_pipapo_avx2: Skip LDMXCSR, we
> don't need a valid MXCSR state").
> 
> Signed-off-by: Eric Biggers <ebiggers@kernel.org>
> ---
>  arch/x86/crypto/aegis128-aesni-glue.c   |  6 +++---
>  arch/x86/crypto/aesni-intel_glue.c      | 10 +++++-----
>  arch/x86/crypto/aria_aesni_avx2_glue.c  |  4 ++--
>  arch/x86/crypto/aria_aesni_avx_glue.c   |  2 +-
>  arch/x86/crypto/aria_gfni_avx512_glue.c |  6 +++---
>  arch/x86/crypto/ecb_cbc_helpers.h       |  2 +-
>  arch/x86/crypto/sm4_aesni_avx_glue.c    |  6 +++---
>  7 files changed, 18 insertions(+), 18 deletions(-)

Applied to https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git/log/?h=libcrypto-next

Reviews and acks appreciated!

- Eric