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Enhance precision and safety in time calculations #54

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merged 2 commits into from
Sep 1, 2024
Merged

Enhance precision and safety in time calculations #54

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83185ab
Add mult_frac() to handle fractional multiplication safely
visitorckw Aug 31, 2024
35e2206
Update semu_timer_clocksource() to use mult_frac()
visitorckw Aug 31, 2024
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17 changes: 15 additions & 2 deletions utils.c
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Original file line number Diff line number Diff line change
Expand Up @@ -19,6 +19,19 @@
#endif
#endif

/* Calculate "x * n / d" without unnecessary overflow or loss of precision.
*
* Reference:
* https://elixir.bootlin.com/linux/v6.10.7/source/include/linux/math.h#L121
*/
static inline uint64_t mult_frac(uint64_t x, uint64_t n, uint64_t d)
{
const uint64_t q = x / d;
const uint64_t r = x % d;

return q * n + r * n / d;
}

void semu_timer_init(semu_timer_t *timer, uint64_t freq)
{
timer->freq = freq;
Expand All @@ -30,12 +43,12 @@ static uint64_t semu_timer_clocksource(uint64_t freq)
#if defined(HAVE_POSIX_TIMER)
struct timespec t;
clock_gettime(CLOCKID, &t);
return (t.tv_sec * freq) + (t.tv_nsec * freq / 1e9);
return t.tv_sec * freq + mult_frac(t.tv_nsec, freq, 1e9);
#elif defined(HAVE_MACH_TIMER)
static mach_timebase_info_data_t t;
if (mach_clk.denom == 0)
(void) mach_timebase_info(&t);
return mach_absolute_time() * freq / t.denom * t.numer;
return mult_frac(mach_absolute_time() * freq, t.numer, t.denom);
#else
return time(0) * freq;
#endif
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