Arm system counter cleanup

Add some description and cleanup code that uses Arm system counter.

Change-Id: Ie1fe00e3e4b5d98867617b7b0184ac526e333c53
Signed-off-by: Brian Brooks <brian.brooks@arm.com>

src/vppinfra/time.c

@@ -142,17 +142,20 @@ done:
 f64
 os_cpu_clock_frequency (void)
 {
+#if defined (__aarch64__)
+  /* The system counter increments at a fixed frequency. It is distributed
+   * to each core which has registers for reading the current counter value
+   * as well as the clock frequency. The system counter is not clocked at
+   * the same frequency as the core. */
+  u32 hz;
+  asm volatile ("mrs %0, cntfrq_el0":"=r" (hz));
+  return (f64) hz;
+#endif
   f64 cpu_freq;
 
   if (clib_cpu_supports_invariant_tsc ())
     return estimate_clock_frequency (1e-3);
 
-#if defined (__aarch64__)
-  u64 tsc;
-  asm volatile ("mrs %0, CNTFRQ_EL0":"=r" (tsc));
-  return (f64) tsc;
-#endif
-
   /* First try /sys version. */
   cpu_freq = clock_frequency_from_sys_filesystem ();
   if (cpu_freq != 0)

src/vppinfra/time.h

@@ -114,6 +114,16 @@ clib_cpu_time_now (void)
   return (u64) lo + ((u64) hi2 << (u64) 32);
 }
 
+#elif defined (__aarch64__)
+always_inline u64
+clib_cpu_time_now (void)
+{
+  u64 vct;
+  /* User access to cntvct_el0 is enabled in Linux kernel since 3.12. */
+  asm volatile ("mrs %0, cntvct_el0":"=r" (vct));
+  return vct;
+}
+
 #elif defined (__arm__)
 #if defined(__ARM_ARCH_8A__)
 always_inline u64
@@ -164,18 +174,6 @@ clib_cpu_time_now (void)
   return ((u64) h << 32) | l;
 }
 
-#elif defined (__aarch64__)
-always_inline u64
-clib_cpu_time_now (void)
-{
-  u64 tsc;
-
-  /* Works on Cavium ThunderX. Other platforms: YMMV */
-  asm volatile ("mrs %0, cntvct_el0":"=r" (tsc));
-
-  return tsc;
-}
-
 #else
 #error "don't know how to read CPU time stamp"