LAPIC timer iinterrupt when TSC reaches the value written to the
IA32_TSC_DEADLINE MSR. To arm or reset the timer in deadline mode, a
single non-serializing MSR write is enough. This is an advance from
the one-shot mode of LAPIC, where timer operated with the FSB
frequency and required two (serialized in case of xAPIC) writes to the
APIC registers.
The LVT_TIMER register value is cached to avoid unneeded writes in the
deadline mode. Unused arguments to specify period (which is passed in
struct lapic as la_timer_period) and interrupt enable (which is always
enabled) are removed from lapic_timer_{oneshot,periodic,deadline}
functions. Instead, special lapic_timer_oneshot_nointr() function for
interrupt-less one-shot calibration is added.
Reviewed by: mav (previous version)
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D5738
ecabd74728