Reading from PMCCNTR causes an undefined instruction exception when
running in PL0 (e.g. as a Linux userspace binary), unless the
PMUSERENR.EN bit is set.
Restructure profile_timestamp() for 32-bit ARM to perform an
availability check on the first invocation, with subsequent
invocations returning zero if PMCCNTR could not be enabled.
Signed-off-by: Michael Brown <mcb30@ipxe.org>
Michael Brown
e0282688c1