tsc(9) - NetBSD Manual Pages

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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 6.1.2                   October 25, 2011                   NetBSD 6.1.2