armv8 - fix os_cpu_clock_frequency for armv8
System timer of ARMv8 runs at a different frequency than the CPU's.
The frequency is fixed, typically in the range 1-50MHz.  It can be
read at CNTFRQ special register.
@@ -147,6 +147,12 @@ os_cpu_clock_frequency (void)
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)
