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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>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.