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Add a delay_tsc() static function for when DELAY() uses the TSC.

Browse Source 
This uses slightly simpler logic than the existing code by using the
full 64-bit counter and thus not having to worry about counter
overflow.

Reviewed by:	kib
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D17850
This commit is contained in:
John Baldwin 2018-11-05 22:51:45 +00:00
parent d3384db33c
commit 3c03efc4ab
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=340168
1 changed files with 30 additions and 18 deletions
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48
sys/x86/x86/delay.c
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@ -51,11 +51,23 @@ __FBSDID("$FreeBSD$");
#include <machine/cpu.h>
#include <x86/init.h>
static u_int
get_tsc(__unused struct timecounter *tc)
static void
delay_tsc(int n)
{
uint64_t end, now;
return (rdtsc32());
/*
* Pin the current thread ensure correct behavior if the TSCs
* on different CPUs are not in sync.
*/
sched_pin();
now = rdtsc();
end = now + tsc_freq * n / 1000000;
do {
cpu_spinwait();
now = rdtsc();
} while (now < end);
sched_unpin();
}
static int
@ -66,22 +78,24 @@ delay_tc(int n)
uint64_t end, freq, now;
u_int last, mask, u;
tc = timecounter;
freq = atomic_load_acq_64(&tsc_freq);
if (tsc_is_invariant && freq != 0) {
func = get_tsc;
mask = ~0u;
} else {
if (tc->tc_quality <= 0)
return (0);
func = tc->tc_get_timecount;
mask = tc->tc_counter_mask;
freq = tc->tc_frequency;
/*
* Only use the TSC if it is P-state invariant. If the TSC is
* not P-state invariant and the CPU is not running at the
* "full" P-state, then the TSC will increment at some rate
* less than tsc_freq and delay_tsc() will wait too long.
*/
if (tsc_is_invariant && tsc_freq != 0) {
delay_tsc(n);
return (1);
}
tc = timecounter;
if (tc->tc_quality <= 0)
return (0);
func = tc->tc_get_timecount;
mask = tc->tc_counter_mask;
freq = tc->tc_frequency;
now = 0;
end = freq * n / 1000000;
if (func == get_tsc)
sched_pin();
last = func(tc) & mask;
do {
cpu_spinwait();
@ -92,8 +106,6 @@ delay_tc(int n)
now += u - last;
last = u;
} while (now < end);
if (func == get_tsc)
sched_unpin();
return (1);
}