The random longs to be pulled by arch_get_random_long() are
prepared in an 4K buffer which is filled from the NIST 800-90
compliant s390 drbg. By default the random long buffer is refilled
256 times before the drbg itself needs a reseed. The reseed of the
drbg is done with 32 bytes fetched from the high quality (but slow)
trng which is assumed to deliver 100% entropy. So the 32 * 8 = 256
bits of entropy are spread over 256 * 4KB = 1MB serving 131072
arch_get_random_long() invocations before reseeded.

How often the 4K random long buffer is refilled with the drbg
before the drbg is reseeded can be adjusted. There is a module
parameter 's390_arch_rnd_long_drbg_reseed' accessible via
  /sys/module/arch_random/parameters/rndlong_drbg_reseed
or as kernel command line parameter
  arch_random.rndlong_drbg_reseed=<value>
This parameter tells how often the drbg fills the 4K buffer before
it is re-seeded by fresh entropy from the trng.
A value of 16 results in reseeding the drbg at every 16 * 4 KB = 64
KB with 32 bytes of fresh entropy pulled from the trng. So a value
of 16 would result in 256 bits entropy per 64 KB.
A value of 256 results in 1MB of drbg output before a reseed of the
drbg is done. So this would spread the 256 bits of entropy among 1MB.
Setting this parameter to 0 forces the reseed to take place every
time the 4K buffer is depleted, so the entropy rises to 256 bits
entropy per 4K or 0.5 bit entropy per arch_get_random_long().  With
setting this parameter to negative values all this effort is
disabled, arch_get_random long() returns false and thus indicating
that the arch_get_random_long() feature is disabled at all.

arch_get_random_long() is used by random.c among others to provide
an initial hash value to be mixed with the entropy pool on every
random data pull. For about 64 bytes read from /dev/urandom there
is one call to arch_get_random_long(). So these additional random
long values count for performance of /dev/urandom with measurable
but low penalty.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Ingo Franzki <ifranzki@linux.ibm.com>
Reviewed-by: Juergen Christ <jchrist@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>