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[arm] Use CNTVCT_EL0 as profiling timestamp
The raw cycle counter at PMCCNTR_EL0 works in qemu but seems to always read as zero on physical hardware (tested on Juno r1 and Cavium ThunderX), even after ensuring that PMCR_EL0.E and PMCNTENSET_EL0.C are both enabled. Use CNTVCT_EL0 instead; this seems to count at a lower resolution (tens of CPU cycles), but is usable for profiling. Signed-off-by: Michael Brown <mcb30@ipxe.org>
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