1OPENSSL_INSTRUMENT_BUS(3)           OpenSSL          OPENSSL_INSTRUMENT_BUS(3)
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NAME

6       OPENSSL_instrument_bus, OPENSSL_instrument_bus2 - instrument references
7       to memory bus
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SYNOPSIS

10        #ifdef OPENSSL_CPUID_OBJ
11        size_t OPENSSL_instrument_bus(int *vector, size_t num);
12        size_t OPENSSL_instrument_bus2(int *vector, size_t num, size_t max);
13        #endif
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DESCRIPTION

16       It was empirically found that timings of references to primary memory
17       are subject to irregular, apparently non-deterministic variations. The
18       subroutines in question instrument these references for purposes of
19       gathering randomness for random number generator. In order to make it
20       bus-bound a 'flush cache line' instruction is used between probes. In
21       addition probes are added to vector elements in atomic or interlocked
22       manner, which should contribute additional noise on multi-processor
23       systems. This also means that vector[num] should be zeroed upon
24       invocation (if you want to retrieve actual probe values).
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26       OPENSSL_instrument_bus() performs num probes and records the number of
27       oscillator cycles every probe took.
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29       OPENSSL_instrument_bus2() on the other hand accumulates consecutive
30       probes with the same value, i.e. in a way it records duration of
31       periods when probe values appeared deterministic. The subroutine
32       performs at most max probes in attempt to fill the vector[num], with
33       max value of 0 meaning "as many as it takes."
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RETURN VALUES

36       Return value of 0 indicates that CPU is not capable of performing the
37       benchmark, either because oscillator counter or 'flush cache line' is
38       not available on current platform. For reference, on x86 'flush cache
39       line' was introduced with the SSE2 extensions.
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41       Otherwise number of recorded values is returned.
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44       Copyright 2011-2018 The OpenSSL Project Authors. All Rights Reserved.
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46       Licensed under the OpenSSL license (the "License").  You may not use
47       this file except in compliance with the License.  You can obtain a copy
48       in the file LICENSE in the source distribution or at
49       <https://www.openssl.org/source/license.html>.
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531.1.1c                            2019-05-28         OPENSSL_INSTRUMENT_BUS(3)
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