aarch64: raise ARM generic-timer frequency ceiling to 2GHz

The generic-timer frequency sanity check rejected anything above 1GHz. Real
server ARM cores run higher: the ampere-1a is at 1GHz and AWS Graviton3
(Neoverse-V1) passes a 1.05GHz generic-timer frequency to the guest, which
tripped the old ceiling and aborted the boot. Raise the ceiling to 2GHz to
cover current cores with headroom while still rejecting a garbage read.

Author: gburd

arch/aarch64/arm-clock.cc

@@ -42,9 +42,12 @@ class arm_clock : public clock {
 
 arm_clock::arm_clock() {
     asm volatile ("mrs %0, cntfrq_el0; isb; " : "=r"(freq_hz) :: "memory");
-    /* spec documents a typical range of 1-50 MHZ,
-     * the ampere-1a however, runs on 1Ghz, so allow up to that frequency */
-    if (freq_hz < 1 * MHZ || freq_hz > 1000 * MHZ) {
+    /* spec documents a typical range of 1-50 MHZ, but real cores run higher:
+     * the ampere-1a runs at 1GHz and AWS Graviton3 (Neoverse-V1) passes a
+     * 1.05GHz generic-timer frequency through to the guest, so the previous
+     * 1GHz ceiling aborted the boot.  Allow up to 2GHz to cover current server
+     * ARM cores with headroom while still catching a garbage read. */
+    if (freq_hz < 1 * MHZ || freq_hz > 2000 * MHZ) {
         debug_early_u64("arm_clock(): read invalid frequency ", freq_hz);
         abort();
     }