Re: [PATCH] AArch64: Check kernel version for SVE ifuncs

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 13/03/24 11:31, Wilco Dijkstra wrote:
> 
> Older Linux kernels may disable SVE after certain syscalls.  Calling the
> SVE-optimized memcpy afterwards will then cause a trap to reenable SVE.
> As a result, applications with a high use of syscalls may run slower with
> the SVE memcpy.  Avoid this by checking the kernel version and enable the
> SVE-optimized memcpy/memmove ifuncs only on Linux kernel 6.2 or newer.
> 
> Passes regress, OK for commit?

How does it handle backports? Parsing kernel version is really not ideal
(and that's why we removed it on previous version), can't kernel provide
this information through any means (as powerpc does for HTM without
suspend, PPC_FEATURE2_HTM_NO_SUSPEND) ?

> 
> ---
> 
> diff --git a/sysdeps/aarch64/cpu-features.h b/sysdeps/aarch64/cpu-features.h
> index 77a782422af1b6e4b2af32bfebfda37874111510..5f2da91ebbd0adafb0d84ec503b0f902f566da5a 100644
> --- a/sysdeps/aarch64/cpu-features.h
> +++ b/sysdeps/aarch64/cpu-features.h
> @@ -71,6 +71,7 @@ struct cpu_features
>    /* Currently, the GLIBC memory tagging tunable only defines 8 bits.  */
>    uint8_t mte_state;
>    bool sve;
> +  bool prefer_sve_ifuncs;
>    bool mops;
>  };
>  
> diff --git a/sysdeps/aarch64/multiarch/init-arch.h b/sysdeps/aarch64/multiarch/init-arch.h
> index c52860efb22d70eb4bdf356781f51c7de8ec67dc..61dc40088f4d9e5e06b57bdc7d54bde1e2a686a4 100644
> --- a/sysdeps/aarch64/multiarch/init-arch.h
> +++ b/sysdeps/aarch64/multiarch/init-arch.h
> @@ -36,5 +36,7 @@
>      MTE_ENABLED ();							      \
>    bool __attribute__((unused)) sve =					      \
>      GLRO(dl_aarch64_cpu_features).sve;					      \
> +  bool __attribute__((unused)) prefer_sve_ifuncs =			      \
> +    GLRO(dl_aarch64_cpu_features).prefer_sve_ifuncs;			      \
>    bool __attribute__((unused)) mops =					      \
>      GLRO(dl_aarch64_cpu_features).mops;
> diff --git a/sysdeps/aarch64/multiarch/memcpy.c b/sysdeps/aarch64/multiarch/memcpy.c
> index d12eccfca51f4bcfef6ccf5aa286edb301e361ac..ce53567dab33c2f00b89b4069235abd4651811a6 100644
> --- a/sysdeps/aarch64/multiarch/memcpy.c
> +++ b/sysdeps/aarch64/multiarch/memcpy.c
> @@ -47,7 +47,7 @@ select_memcpy_ifunc (void)
>      {
>        if (IS_A64FX (midr))
>  	return __memcpy_a64fx;
> -      return __memcpy_sve;
> +      return prefer_sve_ifuncs ? __memcpy_sve : __memcpy_generic;
>      }
>  
>    if (IS_THUNDERX (midr))
> diff --git a/sysdeps/aarch64/multiarch/memmove.c b/sysdeps/aarch64/multiarch/memmove.c
> index 2081eeb4d40e0240e67a7b26b64576f44eaf18e3..fe95037be391896c7670ef606bf4d3ba7dfb6a00 100644
> --- a/sysdeps/aarch64/multiarch/memmove.c
> +++ b/sysdeps/aarch64/multiarch/memmove.c
> @@ -47,7 +47,7 @@ select_memmove_ifunc (void)
>      {
>        if (IS_A64FX (midr))
>  	return __memmove_a64fx;
> -      return __memmove_sve;
> +      return prefer_sve_ifuncs ? __memmove_sve : __memmove_generic;
>      }
>  
>    if (IS_THUNDERX (midr))
> diff --git a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> index b1a3f673f067280bdacfddd92723a81e418023e5..13b02c45df80b493516b3c9d4acbbbffaa47af92 100644
> --- a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> +++ b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> @@ -21,6 +21,7 @@
>  #include <sys/auxv.h>
>  #include <elf/dl-hwcaps.h>
>  #include <sys/prctl.h>
> +#include <sys/utsname.h>
>  #include <dl-tunables-parse.h>
>  
>  #define DCZID_DZP_MASK (1 << 4)
> @@ -62,6 +63,41 @@ get_midr_from_mcpu (const struct tunable_str_t *mcpu)
>    return UINT64_MAX;
>  }
>  
> +/* Parse kernel version without calling any library functions.
> +   Allow 2 digits for kernel version and 3 digits for major version,
> +   separated by '.': "kk.mmm.".
> +   Return kernel version * 1000 + major version, or -1 on failure.  */
> +
> +static inline int
> +kernel_version (void)
> +{
> +  struct utsname buf;
> +  const char *p = &buf.release[0];
> +  int kernel = 0;
> +  int major = 0;
> +
> +  if (__uname (&buf) < 0)
> +    return -1;
> +
> +  if (*p >= '0' && *p <= '9')
> +    kernel = (kernel * 10) + *p++ - '0';
> +  if (*p >= '0' && *p <= '9')
> +    kernel = (kernel * 10) + *p++ - '0';
> +  if (*p != '.')
> +    return -1;
> +  p++;
> +  if (*p >= '0' && *p <= '9')
> +    major = (major * 10) + *p++ - '0';
> +  if (*p >= '0' && *p <= '9')
> +    major = (major * 10) + *p++ - '0';
> +  if (*p >= '0' && *p <= '9')
> +    major = (major * 10) + *p++ - '0';
> +  if (*p != '.' && *p != '\0')
> +    return -1;
> +
> +  return kernel * 1000 + major;
> +}
> +
>  static inline void
>  init_cpu_features (struct cpu_features *cpu_features)
>  {
> @@ -126,6 +162,10 @@ init_cpu_features (struct cpu_features *cpu_features)
>    /* Check if SVE is supported.  */
>    cpu_features->sve = GLRO (dl_hwcap) & HWCAP_SVE;
>  
> +  /* Prefer using SVE in string ifuncs from Linux 6.2 onwards.  */
> +  cpu_features->prefer_sve_ifuncs =
> +    cpu_features->sve && kernel_version () >= 6002;
> +
>    /* Check if MOPS is supported.  */
>    cpu_features->mops = GLRO (dl_hwcap2) & HWCAP2_MOPS;
>  }
>