hwpmc: initialize arm64 counter/interrupt state

Performance counters and overflow interrupts are assumed to be disabled
by default, but this is not guaranteed. Ensure we disable both during
per-cpu initialization, before enabling the PMU. Otherwise, some systems
(such as the Ampere eMAG) would experience an interrupt storm upon
loading the hwpmc module.

Reviewed by:	br
MFC after:	5 days
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D32854

sys/dev/hwpmc/hwpmc_arm64.c

@@ -483,6 +483,16 @@ arm64_pcpu_init(struct pmc_mdep *md, int cpu)
 		pc->pc_hwpmcs[i + first_ri] = phw;
 	}
 
+	/*
+	 * Disable all counters and overflow interrupts. Upon reset they are in
+	 * an undefined state.
+	 *
+	 * Don't issue an isb here, just wait for the one in arm64_pmcr_write()
+	 * to make the writes visible.
+	 */
+	WRITE_SPECIALREG(pmcntenclr_el0, 0xffffffff);
+	WRITE_SPECIALREG(pmintenclr_el1, 0xffffffff);
+
 	/* Enable unit */
 	pmcr = arm64_pmcr_read();
 	pmcr |= PMCR_E;