RFR: 8283199: Linux os::cpu_microcode_revision() stalls cold startup

[Aleksey Shipilev]

Noticed this when staring at timeline profiles for JVM startup. If you run any small startup workload on Linux, there is a significant time gap where JVM does nothing. I pinpointed that to fopen of `/proc/cpuinfo` due to [JDK-8249672](https://bugs.openjdk.java.net/browse/JDK-8249672). 

This *does not* reproduce if you run startup workloads continuously, as it looks as if `/proc/cpuinfo` is cached for about a second, and stalls reads after that, I suspect for CPU info updates, like frequency.

Observe:


$ for I in `seq 1 3`; do sleep 1; time build/linux-x86_64-server-release/images/jdk/bin/java -Xms128m -Xmx128m -version > /dev/null 2>&1 ; done

real	0m0.046s
real	0m0.045s
real	0m0.046s


Without the sleep:


$ for I in `seq 1 3`; do time build/linux-x86_64-server-release/images/jdk/bin/java -Xms128m -Xmx128m -version > /dev/null 2>&1 ; done

real	0m0.026s
real	0m0.026s
real	0m0.023s


There is another way to do this: read `/sys/devices/system/cpu/cpu0/microcode/version`, this is what this patch does. With `sleep 1`:


$ for I in `seq 1 3`; do sleep 1; time build/linux-x86_64-server-release/images/jdk/bin/java -Xms128m -Xmx128m -version > /dev/null 2>&1 ; done

real	0m0.025s
real	0m0.024s
real	0m0.024s


Which means it improves startup time from ~45ms to ~25ms, or about 1.8x!

It raises a bigger question if VM features string should be generated on startup, but it seems to take negligible time otherwise, see [JDK-8283200](https://bugs.openjdk.java.net/browse/JDK-8283200). Given [JDK-8249672](https://bugs.openjdk.java.net/browse/JDK-8249672) had been backported to update releases, let's do a pointed fix for this regression first.

Additional testing:
 - [x] Linux x86_64 fastdebug `tier1`
 - [x] Eyeballing reported microcode number with/without the fix -- same thing reported

-------------

Commit messages:
 - Add comment
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/7825/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7825&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283199
  Stats: 28 lines in 1 file changed: 19 ins; 1 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7825.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7825/head:pull/7825

PR: https://git.openjdk.java.net/jdk/pull/7825