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Skip TSC calibration if exact value known

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It's possible that the "early" TSC calibration gave us a value which
is known to be exact; in that case, skip the later re-calibration.

MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D33695
This commit is contained in:
Colin Percival 2021-12-30 11:43:15 -08:00
parent 324150d6da
commit 9cb3288287
1 changed files with 5 additions and 0 deletions
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5
sys/x86/x86/tsc.c
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@ -59,6 +59,7 @@ __FBSDID("$FreeBSD$");
uint64_t tsc_freq;
int tsc_is_invariant;
int tsc_perf_stat;
static int tsc_early_calib_exact;
static eventhandler_tag tsc_levels_tag, tsc_pre_tag, tsc_post_tag;
@ -133,6 +134,7 @@ tsc_freq_vmware(void)
tsc_freq = regs[0] | ((uint64_t)regs[1] << 32);
}
tsc_is_invariant = 1;
tsc_early_calib_exact = 1;
}
/*
@ -709,6 +711,8 @@ tsc_calibrate(void)
if (tsc_disabled)
return;
if (tsc_early_calib_exact)
goto calibrated;
tc = atomic_load_ptr(&timecounter);
@ -739,6 +743,7 @@ tsc_calibrate(void)
freq_khz = tc->tc_frequency * (tsc_end - tsc_start) / (t_end - t_start);
tsc_update_freq(freq_khz);
calibrated:
tc_init(&tsc_timecounter);
set_cputicker(rdtsc, tsc_freq, !tsc_is_invariant);
}