[glibc] AArch64: Check kernel version for SVE ifuncs

[glibc] AArch64: Check kernel version for SVE ifuncs

[Wilco Dijkstra]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=2e94e2f5d2bf2de124c8ad7da85463355e54ccb2

commit 2e94e2f5d2bf2de124c8ad7da85463355e54ccb2
Author: Wilco Dijkstra <wilco.dijkstra@arm.com>
Date:   Thu Mar 21 16:48:33 2024 +0000

    AArch64: Check kernel version for SVE ifuncs
    
    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.
    
    Tested-by: Florian Weimer <fweimer@redhat.com>
    Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>

Diff:
---
 sysdeps/aarch64/cpu-features.h                 |  1 +
 sysdeps/aarch64/multiarch/init-arch.h          |  2 ++
 sysdeps/aarch64/multiarch/memcpy.c             |  2 +-
 sysdeps/aarch64/multiarch/memmove.c            |  2 +-
 sysdeps/unix/sysv/linux/aarch64/cpu-features.c | 48 ++++++++++++++++++++++++++
 5 files changed, 53 insertions(+), 2 deletions(-)

diff --git a/sysdeps/aarch64/cpu-features.h b/sysdeps/aarch64/cpu-features.h
index 77a782422a..5f2da91ebb 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 c52860efb2..61dc40088f 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 d12eccfca5..ce53567dab 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 2081eeb4d4..fe95037be3 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 b1a3f673f0..c0b047bc0d 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;
 }