CVS commit: src/sys

["SAITOH Masanobu" <msaitoh%netbsd.org@localhost>]

Module Name:    src
Committed By:   msaitoh
Date:           Mon Jun 15 09:09:24 UTC 2020

Modified Files:
        src/sys/arch/amd64/amd64: cpufunc.S
        src/sys/arch/i386/i386: cpufunc.S
        src/sys/arch/x86/include: cpu_counter.h cpufunc.h
        src/sys/arch/x86/x86: cpu.c hyperv.c tsc.c tsc.h
        src/sys/rump/librump/rumpkern/arch/x86: rump_x86_cpu_counter.c

Log Message:
Serialize rdtsc using with lfence, mfence or cpuid to read TSC more precisely.

 x86/x86/tsc.c rev. 1.67 reduced cache problem and got big improvement, but it
 still has room. I measured the effect of lfence, mfence, cpuid and rdtscp.
The impact to TSC skew and/or drift is:

        AMD:   mfence > rdtscp > cpuid > lfence-serialize > lfence = nomodify
        Intel: lfence > rdtscp > cpuid > nomodify

So, mfence is the best on AMD and lfence is the best on Intel. If it has no
SSE2, we can use cpuid.

NOTE:
  - An AMD's document says DE_CFG_LFENCE_SERIALIZE bit can be used for
    serializing, but it's not so good.
  - On Intel i386(not amd64), it seems the improvement is very little.
  - rdtscp instruct can be used as serializing instruction + rdtsc, but
    it's not good as [lm]fence. Both Intel and AMD's document say that
    the latency of rdtscp is bigger than rdtsc, so I suspect the difference
    of the result comes from it.


To generate a diff of this commit:
cvs rdiff -u -r1.60 -r1.61 src/sys/arch/amd64/amd64/cpufunc.S
cvs rdiff -u -r1.46 -r1.47 src/sys/arch/i386/i386/cpufunc.S
cvs rdiff -u -r1.6 -r1.7 src/sys/arch/x86/include/cpu_counter.h
cvs rdiff -u -r1.40 -r1.41 src/sys/arch/x86/include/cpufunc.h
cvs rdiff -u -r1.193 -r1.194 src/sys/arch/x86/x86/cpu.c
cvs rdiff -u -r1.9 -r1.10 src/sys/arch/x86/x86/hyperv.c
cvs rdiff -u -r1.50 -r1.51 src/sys/arch/x86/x86/tsc.c
cvs rdiff -u -r1.6 -r1.7 src/sys/arch/x86/x86/tsc.h
cvs rdiff -u -r1.1 -r1.2 \
    src/sys/rump/librump/rumpkern/arch/x86/rump_x86_cpu_counter.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.