Re: [PATCH v2] AArch64: Check kernel version for SVE ifuncs

[Szabolcs Nagy <Szabolcs.Nagy@arm.com>]

The 03/18/2024 14:14, Wilco Dijkstra wrote:
> 
> v2: Add __LINUX_KERNEL_VERSION #ifdefs, improve parser, use 8:8:8 format.
> 
> Old Linux kernels disable SVE after every system call.  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.  This is true for kernels between 4.15.0 and before 6.2.0,
> except for 5.14.0 which was patched.  Avoid this by checking the kernel
> version and selecting the SVE ifunc on modern kernels.
> 
> Parse the kernel version reported by uname() into a 24-bit kernel.major.minor
> value without calling any library functions.  If uname() is not supported or
> if the version format is not recognized, assume the kernel is modern.
> 
> Passes regress, OK for commit?

OK to commit. (clearly a hack but what can we do..)

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>



> 
> ---
> 
> 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..c0b047bc0dbeae428c89e12688b7d802e4cb3a43 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 +63,46 @@ get_midr_from_mcpu (const struct tunable_str_t *mcpu)
>    return UINT64_MAX;
>  }
> 
> +#if __LINUX_KERNEL_VERSION < 0x060200
> +
> +/* Return true if we prefer using SVE in string ifuncs.  Old kernels disable
> +   SVE after every system call which results in unnecessary traps if memcpy
> +   uses SVE.  This is true for kernels between 4.15.0 and before 6.2.0, except
> +   for 5.14.0 which was patched.  For these versions return false to avoid using
> +   SVE ifuncs.
> +   Parse the kernel version into a 24-bit kernel.major.minor value without
> +   calling any library functions.  If uname() is not supported or if the version
> +   format is not recognized, assume the kernel is modern and return true.  */
> +
> +static inline bool
> +prefer_sve_ifuncs (void)
> +{
> +  struct utsname buf;
> +  const char *p = &buf.release[0];
> +  int kernel = 0;
> +  int val;
> +
> +  if (__uname (&buf) < 0)
> +    return true;
> +
> +  for (int shift = 16; shift >= 0; shift -= 8)
> +    {
> +      for (val = 0; *p >= '0' && *p <= '9'; p++)
> +       val = val * 10 + *p - '0';
> +      kernel |= (val & 255) << shift;
> +      if (*p++ != '.')
> +       break;
> +    }
> +
> +  if (kernel >= 0x060200 || kernel == 0x050e00)
> +    return true;
> +  if (kernel >= 0x040f00)
> +    return false;
> +  return true;
> +}
> +
> +#endif
> +
>  static inline void
>  init_cpu_features (struct cpu_features *cpu_features)
>  {
> @@ -126,6 +167,13 @@ init_cpu_features (struct cpu_features *cpu_features)
>    /* Check if SVE is supported.  */
>    cpu_features->sve = GLRO (dl_hwcap) & HWCAP_SVE;
> 
> +  cpu_features->prefer_sve_ifuncs = cpu_features->sve;
> +
> +#if __LINUX_KERNEL_VERSION < 0x060200
> +  if (cpu_features->sve)
> +    cpu_features->prefer_sve_ifuncs = prefer_sve_ifuncs ();
> +#endif
> +
>    /* Check if MOPS is supported.  */
>    cpu_features->mops = GLRO (dl_hwcap2) & HWCAP2_MOPS;
>  }
>