1=pod
2
3=head1 NAME
4
5OPENSSL_instrument_bus, OPENSSL_instrument_bus2 - instrument references to memory bus
6
7=head1 SYNOPSIS
8
9 #ifdef OPENSSL_CPUID_OBJ
10 size_t OPENSSL_instrument_bus(unsigned int *vector, size_t num);
11 size_t OPENSSL_instrument_bus2(unsigned int *vector, size_t num, size_t max);
12 #endif
13
14=head1 DESCRIPTION
15
16It was empirically found that timings of references to primary memory
17are subject to irregular, apparently non-deterministic variations. The
18subroutines in question instrument these references for purposes of
19gathering randomness for random number generator. In order to make it
20bus-bound a 'flush cache line' instruction is used between probes. In
21addition probes are added to B<vector> elements in atomic or
22interlocked manner, which should contribute additional noise on
23multi-processor systems. This also means that B<vector[num]> should be
24zeroed upon invocation (if you want to retrieve actual probe values).
25
26OPENSSL_instrument_bus() performs B<num> probes and records the number of
27oscillator cycles every probe took.
28
29OPENSSL_instrument_bus2() on the other hand B<accumulates> consecutive
30probes with the same value, i.e. in a way it records duration of
31periods when probe values appeared deterministic. The subroutine
32performs at most B<max> probes in attempt to fill the B<vector[num]>,
33with B<max> value of 0 meaning "as many as it takes."
34
35=head1 RETURN VALUES
36
37Return value of 0 indicates that CPU is not capable of performing the
38benchmark, either because oscillator counter or 'flush cache line' is
39not available on current platform. For reference, on x86 'flush cache
40line' was introduced with the SSE2 extensions.
41
42Otherwise number of recorded values is returned.
43
44=head1 COPYRIGHT
45
46Copyright 2011-2018 The OpenSSL Project Authors. All Rights Reserved.
47
48Licensed under the Apache License 2.0 (the "License").  You may not use
49this file except in compliance with the License.  You can obtain a copy
50in the file LICENSE in the source distribution or at
51L<https://www.openssl.org/source/license.html>.
52
53=cut
54