On x86 platforms with the intrinsic, rdrand is a deterministic bit generator
(AES-CTR) seeded from an entropic source. On x86 platforms with rdseed, it
is something closer to the upstream entropic source. (There is more nuance;
a block diagram is provided in [1].)
On devices with rdrand and without rdseed, there is no good intrinsic for
acecssing the good entropic soure directly. However, the DRBG is guaranteed
to reseed every 8 kB on these platforms. As a conservative option, on such
hardware we can read an extra 7.99kB samples every time we want a sample
from an independent seed.
As one can imagine, this drastically slows the effective read rate of
RDRAND (a factor of 1024 on amd64 and 2048 on ia32). Microbenchmarks on AMD
Zen (has RDSEED) show an RDRAND rate of 25 MB/s and Intel Haswell (no
RDSEED) show RDRAND of 170 MB/s. This would reduce the read rate on Haswell
to ~170 kB/s (at 100% CPU). random(4)'s harvestq thread periodically
"feeds" from pure sources in amounts of 128-1024 bytes. On Haswell,
enabling this feature increases the CPU time of RDRAND in each "feed" from
approximately 0.7-6 µs to 0.7-6 ms.
Because there is some performance penalty to this more conservative option,
a knob is provided to enable the change. The change does not affect
platforms with RDSEED.
[1]: https://software.intel.com/en-us/articles/intel-digital-random-number-generator-drng-software-implementation-guide#inpage-nav-4-2
Approved by: csprng(delphij, markm)
Differential Revision: https://reviews.freebsd.org/D22455
cb285f7c7c