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dtrace/arm64: Fix dtrace_gethrtime()

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This routine returns a monotonic count of the number of nanoseconds elapsed
since the previous call.  On arm64 it uses the generic system timer.  The
implementation multiplies the counter value by 10**9 then divides by the counter
frequency, but this multiplication can overflow.  This can result in trace
records with non-monotonic timestamps, which breaks libdtrace's temporal
ordering algorithm.

An easy fix is to reverse the order of operations, since the counter frequency
will in general be smaller than 10**9.  (In fact, it's mandated to be 1Ghz in
ARMv9, which makes life simple.)  However, this can give a fair bit of error.
Adopt the calculation used on amd64, with tweaks to handle frequencies as low as
1MHz: the ARM generic timer documentation suggests that ARMv8 timers are
typically in the 1MHz-50MHz range, which is true on arm64 systems that I have
access to.

MFC after:	2 weeks
Sponsored by:	Innovate UK
Differential Revision:	https://reviews.freebsd.org/D49244

(cherry picked from commit 36ae5ce2f2fda35763c2655a19bf1b0ee22fdf3c)
This commit is contained in:
Mark Johnston 2025-03-09 23:01:13 -04:00
parent 1cbe878164
commit 294cda7e4e
1 changed files with 31 additions and 2 deletions
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33
sys/cddl/dev/dtrace/aarch64/dtrace_subr.c
View file

@ -151,6 +151,32 @@ dtrace_sync(void)
dtrace_xcall(DTRACE_CPUALL, (dtrace_xcall_t)dtrace_sync_func, NULL);
}
static uint64_t nsec_scale;
#define SCALE_SHIFT 25
/*
* Choose scaling factors which let us convert a cntvct_el0 value to nanoseconds
* without overflow, as in the amd64 implementation.
*
* Documentation for the ARM generic timer states that typical counter
* frequencies are in the range 1Mhz-50Mhz; in ARMv9 the frequency is fixed at
* 1GHz. The lower bound of 1MHz forces the shift to be at most 25 bits. At
* that frequency, the calculation (hi * scale) << (32 - shift) will not
* overflow for over 100 years, assuming that the counter value starts at 0 upon
* boot.
*/
static void
dtrace_gethrtime_init(void *arg __unused)
{
uint64_t freq;
freq = READ_SPECIALREG(cntfrq_el0);
nsec_scale = ((uint64_t)NANOSEC << SCALE_SHIFT) / freq;
}
SYSINIT(dtrace_gethrtime_init, SI_SUB_DTRACE, SI_ORDER_ANY,
dtrace_gethrtime_init, NULL);
/*
* DTrace needs a high resolution time function which can be called from a
* probe context and guaranteed not to have instrumented with probes itself.
@ -161,10 +187,13 @@ uint64_t
dtrace_gethrtime(void)
{
uint64_t count, freq;
uint32_t lo, hi;
count = READ_SPECIALREG(cntvct_el0);
freq = READ_SPECIALREG(cntfrq_el0);
return ((1000000000UL * count) / freq);
lo = count;
hi = count >> 32;
return (((lo * nsec_scale) >> SCALE_SHIFT) +
((hi * nsec_scale) << (32 - SCALE_SHIFT)));
}
/*