NAME

       OPENSSL_instrument_bus, OPENSSL_instrument_bus2 - instrument references to memory bus

SYNOPSIS

        #ifdef OPENSSL_CPUID_OBJ
        size_t OPENSSL_instrument_bus(unsigned int *vector, size_t num);
        size_t OPENSSL_instrument_bus2(unsigned int *vector, size_t num, size_t max);
        #endif

DESCRIPTION

       It was empirically found that timings of references to primary memory are subject to
       irregular, apparently non-deterministic variations. The subroutines in question instrument
       these references for purposes of gathering randomness for random number generator. In
       order to make it bus-bound a 'flush cache line' instruction is used between probes. In
       addition probes are added to vector elements in atomic or interlocked manner, which should
       contribute additional noise on multi-processor systems. This also means that vector[num]
       should be zeroed upon invocation (if you want to retrieve actual probe values).

       OPENSSL_instrument_bus() performs num probes and records the number of oscillator cycles
       every probe took.

       OPENSSL_instrument_bus2() on the other hand accumulates consecutive probes with the same
       value, i.e. in a way it records duration of periods when probe values appeared
       deterministic. The subroutine performs at most max probes in attempt to fill the
       vector[num], with max value of 0 meaning "as many as it takes."

RETURN VALUES

       Return value of 0 indicates that CPU is not capable of performing the benchmark, either
       because oscillator counter or 'flush cache line' is not available on current platform. For
       reference, on x86 'flush cache line' was introduced with the SSE2 extensions.

       Otherwise number of recorded values is returned.

COPYRIGHT

       Copyright 2011-2021 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (the "License").  You may not use this file except
       in compliance with the License.  You can obtain a copy in the file LICENSE in the source
       distribution or at <https://www.openssl.org/source/license.html>.