tsc(9) - NetBSD Manual Pages

TSC(9)               NetBSD/x86 Kernel Developer's Manual               TSC(9)


NAME
tsc -- Time Stamp Counter
SYNOPSIS
#include <x86/x86/tsc.h> uint64_t rdtsc(void); void tsc_tc_init(void); void tsc_sync_ap(struct cpu_info *ci); void tsc_sync_bp(struct cpu_info *ci); void tsc_sync_drift(int64_t drift);
DESCRIPTION
The time stamp counter (TSC) is a hardware counter found in all contempo- rary x86 processors. The counter is implemented as a 64-bit model-spe- cific register (MSR) that is incremented at every clock cycle. The RDTSC (``read time stamp counter'') register has been present since the origi- nal Pentium. Already because of the access method, TSC has traditionally provided a low-overhead and high-resolution way to obtain CPU timing information. Recently, however, this reliability has been undermined by such factors as system sleep states, CPU ``hotplugging'', ``hibernation'', and CPU frequency scaling. These potential new sources of unreliability are easily understandable when one recalls that the counter measures cycles and not ``time''. Com- paring the cycle counts only makes sense when the clock frequency is sta- ble; to convert the cycle counts to time units, a general equation would be: ``seconds = cycles / frequency in Hz''. The use of TSC as a source of high-resolution timing can be thus discouraged. But the basic premise is still guaranteed: TSC is a monotonically increasing counter.
FUNCTIONS
rdtsc() The rdtsc() function returns the value read from RDTSC. tsc_tc_init() The tsc_tc_init() function initializes the TSC as a timecounter(9). The function is called early in the boot process when the proces- sors attach. tsc_sync_ap(ci) The tsc_sync_ap() function synchronizes the counter for the boot processor (BP). The supplied ci must refer to the BP itself. The tsc interface takes internally care of such issues as out-of-order execution, where instructions are not necessarily performed in the order of execution, possibly causing a misleading cycle count. tsc_sync_bp(ci) The tsc_sync_bp() function synchronize the counter for the applica- tion processor ci. Interrupts must be off at machine-level when the function is called. It is necessary to call both tsc_sync_ap() and tsc_sync_bp() during the boot, but additional synchronization may be required also dur- ing runtime. As an example, the TSC needs to be synchronized for all processors when the system resumes from an acpi(4) sleep state. tsc_sync_drift(drift) Finally, the tsc_sync_drift() function records drift, measured in clock cycles. This is called when the APs attach.
SEE ALSO
gettimeofday(2), hpet(4), hz(9), rdmsr(9), timecounter(9) NetBSD 6.0 October 25, 2011 NetBSD 6.0

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