Currently the VDSO does not handle
     clock_gettime( CLOCK_MONOTONIC_RAW, &ts )
  on Intel / AMD - it calls
  for this clock, which issues a syscall, having an unacceptably high
  latency (minimum measurable time or time between measurements)
  of 300-700ns on 2 2.8-3.9ghz Haswell x86_64 Family'_'Model : 06_3C
  machines under various versions of Linux.

  Sometimes, particularly when correlating elapsed time to performance
  counter values, user-space  code needs to know elapsed time from the
  perspective of the CPU no matter how "hot" / fast or "cold" / slow it
  might be running wrt NTP / PTP "real" time; when code needs this,
  the latencies associated with a syscall are often unacceptably high.

  I reported this as Bug #198161 :
  and in previous posts with subjects matching 'CLOCK_MONOTONIC_RAW' .
  This patch handles CLOCK_MONOTONIC_RAW clock_gettime() in the VDSO ,
  by exporting the raw clock calibration, last cycles, last xtime_nsec,
  and last raw_sec value in the vsyscall_gtod_data during vsyscall_update() .

  Now the new do_monotonic_raw() function in the vDSO has a latency of @ 24ns
  on average, and the test program:
  succeeds with arguments: '-c 4 -t 120' or any arbitrary -t value.

  The patch is against Linus' latest 4.16-rc5 tree,
  current HEAD of :

  This patch affects only files:

  There are 3 patches in the series :

   Patch #1 makes the VDSO handle clock_gettime(CLOCK_MONOTONIC_RAW) with 

   Patch #2 makes the VDSO handle clock_gettime(CLOCK_MONOTONIC_RAW) with a new 
rdtscp() function in msr.h

   Patch #3 makes the VDSO export TSC calibration data via a new function in 
the vDSO: 
               unsigned int __vdso_linux_tsc_calibration ( struct 
linux_tsc_calibration *tsc_cal )
            that user code can optionally call.

   Patches #2 & #3 should be considered "optional" .

   Patch #2 makes clock_gettime(CLOCK_MONOTONIC_RAW) calls have @ half the 
   of clock_gettime(CLOCK_MONOTONIC) calls.

   I think something like Patch #3 is necessary to export TSC calibration data 
to user-space TSC readers.

Best Regards,
Jason Vas Dias

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