[PATCH] AArch64: Check kernel version for SVE ifuncs

[Wilco Dijkstra <Wilco.Dijkstra@arm.com>]

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?

---

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;
 }