tunables, aarch64: New tunable to override cpu

Add a new tunable (glibc.tune.cpu) to override CPU identification on
aarch64.  This is useful in two cases: one where it is desirable to
pretend to be another CPU for purposes of testing or because routines
written for that CPU are beneficial for specific workloads and second
where the underlying kernel does not support emulation of MRS to get
the MIDR of the CPU.

	* elf/dl-tunables.h (tunable_is_name): Move from...
	* elf/dl-tunables.c (is_name): ... here.
	(parse_tunables, __tunables_init): Adjust.
	* manual/tunables.texi: Document glibc.tune.cpu.
	* sysdeps/aarch64/dl-tunables.list: New file.
	* sysdeps/unix/sysv/linux/aarch64/cpu-features.c (struct
	cpu_list): New type.
	(cpu_list): New list of CPU names and their MIDR.
	(get_midr_from_mcpu): New function.
	(init_cpu_features): Override MIDR if necessary.

1 files changed, 41 insertions, 6 deletions

diff --git a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
index ef6eecd..47c7d97 100644
--- a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
+++ b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
@@ -20,18 +20,53 @@
 #include <sys/auxv.h>
 #include <elf/dl-hwcaps.h>
 
+#if HAVE_TUNABLES
+struct cpu_list
+{
+  const char *name;
+  uint64_t midr;
+};
+
+static struct cpu_list cpu_list[] = {
+      {"thunderxt88",	0x430F0A10},
+      {"generic", 	0x0}
+};
+
+static uint64_t
+get_midr_from_mcpu (const char *mcpu)
+{
+  for (int i = 0; i < sizeof (cpu_list) / sizeof (struct cpu_list); i++)
+    if (tunable_is_name (mcpu, cpu_list[i].name) == 0)
+      return cpu_list[i].midr;
+
+  return UINT64_MAX;
+}
+#endif
+
 static inline void
 init_cpu_features (struct cpu_features *cpu_features)
 {
   uint64_t hwcap_mask = GET_HWCAP_MASK();
   uint64_t hwcap = GLRO (dl_hwcap) & hwcap_mask;
 
-  if (hwcap & HWCAP_CPUID)
+  register uint64_t midr = UINT64_MAX;
+
+#if HAVE_TUNABLES
+  /* Get the tunable override.  */
+  const char *mcpu = TUNABLE_GET (glibc, tune, mcpu, const char *, NULL);
+  if (mcpu != NULL)
+    midr = get_midr_from_mcpu (mcpu);
+#endif
+
+  /* If there was no useful tunable override, query the MIDR if the kernel
+     allows it.  */
+  if (midr == UINT64_MAX)
     {
-      register uint64_t id = 0;
-      asm volatile ("mrs %0, midr_el1" : "=r"(id));
-      cpu_features->midr_el1 = id;
+      if (hwcap & HWCAP_CPUID)
+	asm volatile ("mrs %0, midr_el1" : "=r"(midr));
+      else
+	midr = 0;
     }
-  else
-    cpu_features->midr_el1 = 0;
+
+  cpu_features->midr_el1 = midr;
 }