x86: Replace RDRAND forced-reseed with simple sanity check

x86: Replace RDRAND forced-reseed with simple sanity check

x86_init_rdrand() was added with 2 goals:

1. Sanity check that the built-in-self-test circuit on the Digital
   Random Number Generator (DRNG) is not complaining.  As RDRAND
   HW self-checks on every invocation, this goal is achieved
   by simply invoking RDRAND and checking its return code.

2. Force a full re-seed of the random number generator.
   This was done out of paranoia to benefit the most un-sophisticated
   DRNG implementation conceivable in the architecture,
   an implementation that does not exist, and unlikely ever will.
   This worst-case full-re-seed is achieved by invoking
   a 64-bit RDRAND 8192 times.

Unfortunately, this worst-case re-seed costs O(1,000us).
Magnifying this cost, it is done from identify_cpu(), which is the
synchronous critical path to bring a processor on-line -- repeated
for every logical processor in the system at boot and resume from S3.

As it is very expensive, and of highly dubious value, we delete the
worst-case re-seed from the kernel.

We keep the 1st goal -- sanity check the hardware, and mark it absent
if it complains.

This change reduces the cost of x86_init_rdrand() by a factor of 1,000x,
to O(1us) from O(1,000us).

Signed-off-by: Len Brown <len.brown@intel.com>
Link: http://lkml.kernel.org/r/058618cc56ec6611171427ad7205e37e377aa8d4.1439738240.git.len.brown@intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>