amd64, i386: set lapic timer mode, mask, divisor once

[Scott Cheloha]

mlarkin@ noted about a month or so ago that setting the lapic timer
mode, mask, and divisor every time we rearm it is unnecessary.  We
only need to configure those registers once during
lapic_timer_trigger().  After that, it is sufficient to set the ICR
when rearming the timer.

While here, add the missing intr_disable/intr_restore wrapper to
lapic_timer_trigger().  Writing multiple registers is not atomic, so
we need to disable interrupts for safety.  Setting the ICR during
lapic_timer_rearm() is atomic, so we don't need to disable interrupts
there.

ok?

Index: amd64/amd64/lapic.c
===================================================================
RCS file: /cvs/src/sys/arch/amd64/amd64/lapic.c,v
retrieving revision 1.65
diff -u -p -r1.65 lapic.c
--- amd64/amd64/lapic.c 10 Nov 2022 08:26:54 -0000      1.65
+++ amd64/amd64/lapic.c 27 Jan 2023 13:58:15 -0000
@@ -431,13 +431,17 @@ lapic_timer_rearm(void *unused, uint64_t
        cycles = (nsecs * lapic_timer_nsec_cycle_ratio) >> 32;
        if (cycles == 0)
                cycles = 1;
-       lapic_timer_oneshot(0, cycles);
+       lapic_writereg(LAPIC_ICR_TIMER, cycles);
 }
 
 void
 lapic_timer_trigger(void *unused)
 {
+       u_long s;
+
+       s = intr_disable();     
        lapic_timer_oneshot(0, 1);
+       intr_restore(s);
 }
 
 /*
Index: i386/i386/lapic.c
===================================================================
RCS file: /cvs/src/sys/arch/i386/i386/lapic.c,v
retrieving revision 1.53
diff -u -p -r1.53 lapic.c
--- i386/i386/lapic.c   6 Dec 2022 01:56:44 -0000       1.53
+++ i386/i386/lapic.c   27 Jan 2023 13:58:15 -0000
@@ -268,13 +268,17 @@ lapic_timer_rearm(void *unused, uint64_t
        cycles = (nsecs * lapic_timer_nsec_cycle_ratio) >> 32;
        if (cycles == 0)
                cycles = 1;
-       lapic_timer_oneshot(0, cycles);
+       i82489_writereg(LAPIC_ICR_TIMER, cycles);
 }
 
 void
 lapic_timer_trigger(void *unused)
 {
+       u_long s;
+
+       s = intr_disable();
        lapic_timer_oneshot(0, 1);
+       intr_restore(s);
 }
 
 /*