POWER9: Is there a way to improve the random number generation on PPC64?

[Gustavo Romero]

Hi,

POWER9 processors introduced a new single instruction to generate a random
number called 'darn' (Deliver A Random Number) [1, 2]. The random number
generator behind this instruction is NIST SP800-90B and SP800-90C compliant and
provides a minimum of 0.5 bits of entropy per bit. That instruction is as simple
as "darn RT, L", where RT is general 64-bit purpose register and L is a 2-bit
operand to select the random number format. One can call 'darn' many times to
obtain a new random number each time.

Initially I think it can help on the improving (throughput) of SecureRandom.generateSeed()
method & friends from JCE (NativePRNG provider). If that holds, so it has to
be done both for Interpreter and JIT.

Currently generateSeed() from NativePRNG basically reads from /dev/random by
default (which blocks from time to time) or /dev/urandom if instructed to do so.
Could somebody please help me to figure out the appropriate place to exploit
such a P9 instruction for interpreted mode, given that code for generateSeed()
is pure Java and behind scenes just opens /dev/random file and reads from
it? For instance, is it correct to exploit it on a C/C++ code and attach that
by means of a JNI?

Finally, for JITed mode, I think that a way to exploit such a feature would be
by matching an specific sub-tree in Ideal Graph and from that emit a `darn`
instruction, however I could not figure one sound sub-tree with known nodes
(AddI, LoadN, Parm, etc) that could be matched for that purpose. How do porters
usually proceed in this case?

Any comments shedding some light on that is much appreciated.

Thanks and best regards,
Gustavo

[1] https://www.docdroid.net/tWT7hjD/powerisa-v30.pdf, p. 79
[2] https://openpowerfoundation.org/?resource_lib=power-isa-version-3-0