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<title>OPENSSL_instrument_bus</title>
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<li><a href="#NAME">NAME</a></li>
<li><a href="#SYNOPSIS">SYNOPSIS</a></li>
<li><a href="#DESCRIPTION">DESCRIPTION</a></li>
<li><a href="#RETURN-VALUES">RETURN VALUES</a></li>
<li><a href="#COPYRIGHT">COPYRIGHT</a></li>
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<h1 id="NAME">NAME</h1>
<p>OPENSSL_instrument_bus, OPENSSL_instrument_bus2 - instrument references to memory bus</p>
<h1 id="SYNOPSIS">SYNOPSIS</h1>
<pre><code> #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</code></pre>
<h1 id="DESCRIPTION">DESCRIPTION</h1>
<p>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 <b>vector</b> elements in atomic or interlocked manner, which should contribute additional noise on multi-processor systems. This also means that <b>vector[num]</b> should be zeroed upon invocation (if you want to retrieve actual probe values).</p>
<p>OPENSSL_instrument_bus() performs <b>num</b> probes and records the number of oscillator cycles every probe took.</p>
<p>OPENSSL_instrument_bus2() on the other hand <b>accumulates</b> 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 <b>max</b> probes in attempt to fill the <b>vector[num]</b>, with <b>max</b> value of 0 meaning "as many as it takes."</p>
<h1 id="RETURN-VALUES">RETURN VALUES</h1>
<p>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.</p>
<p>Otherwise number of recorded values is returned.</p>
<h1 id="COPYRIGHT">COPYRIGHT</h1>
<p>Copyright 2011-2018 The OpenSSL Project Authors. All Rights Reserved.</p>
<p>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 <a href="https://www.openssl.org/source/license.html">https://www.openssl.org/source/license.html</a>.</p>
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