| 1 | =pod
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| 2 |
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| 3 | =head1 NAME
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| 4 |
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| 5 | OPENSSL_instrument_bus, OPENSSL_instrument_bus2 - instrument references to memory bus
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| 6 |
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| 7 | =head1 SYNOPSIS
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| 8 |
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| 9 | #ifdef OPENSSL_CPUID_OBJ
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| 10 | size_t OPENSSL_instrument_bus(unsigned int *vector, size_t num);
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| 11 | size_t OPENSSL_instrument_bus2(unsigned int *vector, size_t num, size_t max);
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| 12 | #endif
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| 13 |
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| 14 | =head1 DESCRIPTION
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| 15 |
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| 16 | It was empirically found that timings of references to primary memory
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| 17 | are subject to irregular, apparently non-deterministic variations. The
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| 18 | subroutines in question instrument these references for purposes of
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| 19 | gathering randomness for random number generator. In order to make it
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| 20 | bus-bound a 'flush cache line' instruction is used between probes. In
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| 21 | addition probes are added to B<vector> elements in atomic or
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| 22 | interlocked manner, which should contribute additional noise on
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| 23 | multi-processor systems. This also means that B<vector[num]> should be
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| 24 | zeroed upon invocation (if you want to retrieve actual probe values).
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| 25 |
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| 26 | OPENSSL_instrument_bus() performs B<num> probes and records the number of
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| 27 | oscillator cycles every probe took.
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| 28 |
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| 29 | OPENSSL_instrument_bus2() on the other hand B<accumulates> consecutive
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| 30 | probes with the same value, i.e. in a way it records duration of
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| 31 | periods when probe values appeared deterministic. The subroutine
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| 32 | performs at most B<max> probes in attempt to fill the B<vector[num]>,
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| 33 | with B<max> value of 0 meaning "as many as it takes."
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| 34 |
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| 35 | =head1 RETURN VALUES
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| 36 |
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| 37 | Return value of 0 indicates that CPU is not capable of performing the
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| 38 | benchmark, either because oscillator counter or 'flush cache line' is
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| 39 | not available on current platform. For reference, on x86 'flush cache
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| 40 | line' was introduced with the SSE2 extensions.
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| 41 |
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| 42 | Otherwise number of recorded values is returned.
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| 43 |
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| 44 | =head1 COPYRIGHT
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| 45 |
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| 46 | Copyright 2011-2021 The OpenSSL Project Authors. All Rights Reserved.
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| 47 |
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| 48 | Licensed under the Apache License 2.0 (the "License"). You may not use
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| 49 | this file except in compliance with the License. You can obtain a copy
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| 50 | in the file LICENSE in the source distribution or at
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| 51 | L<https://www.openssl.org/source/license.html>.
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| 52 |
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| 53 | =cut
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