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imx8: set virtual timer offset to zero for all cores

we want CNTVCT_EL0 to be the same on all cores,
so we set the offset the same so it effectively
becomes the system counter.

--- a/sys/src/9/imx8/clock.c

+++ b/sys/src/9/imx8/clock.c

@@ -41,6 +41,9 @@

 		freq = sysrd(CNTFRQ_EL0);

 		print("timer frequency %lld Hz\n", freq);

 	}

+	m->cpuhz = freq;

+	m->cpumhz = (freq + Mhz/2 - 1) / Mhz;

+	m->cyclefreq = freq;

 	intrenable(IRQcntpns, localclockintr, nil, BUSUNKNOWN, "clock");

 }

--- a/sys/src/9/imx8/l.s

+++ b/sys/src/9/imx8/l.s

@@ -96,6 +96,10 @@

 	MSR	R0, MDCR_EL2

 	ISB	$SY

+	/* set virtual timer offset to zero */

+	MOV	$0, R0

+	MSR	R0, CNTVOFF_EL2

+

 	/* HCR = RW, HCD, SWIO, BSU, FB */

 	MOVWU	$(1<<31 | 1<<29 | 1<<2 | 0<<10 | 0<<9), R0

 	MSR	R0, HCR_EL2

--- a/sys/src/9/imx8/sysreg.h

+++ b/sys/src/9/imx8/sysreg.h

@@ -38,6 +38,7 @@

 #define CNTP_TVAL_EL0			SYSREG(3,3,14,2,0)

 #define CNTP_CTL_EL0			SYSREG(3,3,14,2,1)

 #define CNTP_CVAL_EL0			SYSREG(3,3,14,2,2)

+#define CNTVOFF_EL2			SYSREG(3,4,14,0,3)

 #define TPIDR_EL0			SYSREG(3,3,13,0,2)

 #define TPIDR_EL1			SYSREG(3,0,13,0,4)