Re: [PATCH 0/3] zstd: probe the CPU for BMI2 support once, not per context

[Eric Biggers]

On Wed, Aug 26, 2026 at 05:25:35AM -0700, Usama Arif wrote:
> zstd picks between BMI2 and generic code paths at runtime, and issues
> CPUID to decide every time a compression or decompression context is set
> up. The answer cannot change while the kernel is running.
> 
> It is not a cold path: squashfs calls zstd_init_dstream() for every block
> it decompresses, and erofs, btrfs, f2fs and crypto/zstd all initialise a
> context per operation. Each probe is two serializing CPUID instructions
> on x86.
> 
> Patch 1 routes ZSTD_initStaticCCtx() through ZSTD_cpuSupportsBmi2()
> instead of open-coding the probe, which also fixes it testing for BMI2
> without BMI1 - the bodies it selects are tagged
> TARGET_ATTRIBUTE("lzcnt,bmi,bmi2"), so both are needed. No CPU in the
> field implements BMI2 without BMI1, so this is latent. Patch 2 skips the
> probe when DYNAMIC_BMI2 is 0, where every consumer ignores the flag
> anyway. Patch 3 caches the result.
> 
> A 4 KiB crypto_acomp benchmark [1] in a one-vCPU KVM guest, twelve boots
> of nine 30,000-operation rounds, median per-round mean over 108 rounds:
> 
>                     compress     decompress
>   unpatched         16,756 ns     3,455 ns
>   patched           13,646 ns     1,002 ns
>                     -3,110 ns    -2,452 ns
>                      (18.6%)      (71.0%)
> 
> The main reason is because CPUID is an unconditional VM exit.
> 
> [1] https://gist.github.com/uarif1/5cf02f0e22c23f0d1b3d84348f12914c
>  
> Usama Arif (3):
>   zstd: use ZSTD_cpuSupportsBmi2() in ZSTD_initStaticCCtx()
>   zstd: skip the BMI2 probe when dynamic BMI2 dispatch is disabled
>   zstd: probe the CPU for BMI2 support only once
> 
>  lib/zstd/common/zstd_internal.h   | 22 ++++++++++++++++++++--
>  lib/zstd/compress/zstd_compress.c |  2 +-
>  2 files changed, 21 insertions(+), 3 deletions(-)

Why not just use cpu_feature_enabled(X86_FEATURE_BMI2), which compiles
down to a static branch?  All these issues are caused by lib/zstd/ using
its own custom CPU feature detection code, instead of the normal CPU
feature detection code that the rest of the kernel uses.

- Eric