huschi.net

OPENSSL_INSTRUMENT_BUS(3)           OpenSSL          OPENSSL_INSTRUMENT_BUS(3)
NAME
       OPENSSL_instrument_bus, OPENSSL_instrument_bus2 - instrument references
       to memory bus
SYNOPSIS
        #ifdef OPENSSL_CPUID_OBJ
        size_t OPENSSL_instrument_bus(int *vector, size_t num);
        size_t OPENSSL_instrument_bus2(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 2011-2018 The OpenSSL Project Authors. All Rights Reserved.
       Licensed under the OpenSSL license (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>;.
1.1.1k                            2021-03-25         OPENSSL_INSTRUMENT_BUS(3)