[HyperV-2016] urandom performance issue...

[Support SimpleRezo]

Hi list!

I was figuring a performance issue of a FreeBSD on HyperV, after:
  - moving it from an Hyper-V 2012 host to a Hyper-V 2016
  - updating to FreeBSD 11.2 (custom kernel, but same issue with GENERIC)
After loosing some times because i was thinking about hardware issue (disk
controller was my first thought...) and testing with "dd
if=/dev/urandom"... I discovered this...

I ran the following test... and quite surprised by the speed result!
$ dd if=/dev/urandom of=/dev/null bs=10M count=3
3+0 records in
3+0 records out
31457280 bytes transferred in 27.960431 secs (1125064 bytes/sec)
The test was also sometimes "freezing" the Hyper-V host!

Looking into "dmesg" i have found this:
random: registering fast source Intel Secure Key RNG
random: fast provider: "Intel Secure Key RNG"

So I rebuilt my KERNEL without "rdrand_rng" (didn't find another way to
disable it...)... and performances went back to normal!

I think this issue should documented on https://wiki.freebsd.org/HyperV !

Regards

--
Clement
SimpleRezo