1 =pod
2 
3 =head1 NAME
4 
5 OPENSSL_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 
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 B<vector> elements in atomic or
22 interlocked manner, which should contribute additional noise on
23 multi-processor systems. This also means that B<vector[num]> should be
24 zeroed upon invocation (if you want to retrieve actual probe values).
25 
26 OPENSSL_instrument_bus() performs B<num> probes and records the number of
27 oscillator cycles every probe took.
28 
29 OPENSSL_instrument_bus2() on the other hand B<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 B<max> probes in attempt to fill the B<vector[num]>,
33 with B<max> value of 0 meaning "as many as it takes."
34 
35 =head1 RETURN VALUES
36 
37 Return value of 0 indicates that CPU is not capable of performing the
38 benchmark, either because oscillator counter or 'flush cache line' is
39 not available on current platform. For reference, on x86 'flush cache
40 line' was introduced with the SSE2 extensions.
41 
42 Otherwise number of recorded values is returned.
43 
44 =head1 COPYRIGHT
45 
46 Copyright 2011-2018 The OpenSSL Project Authors. All Rights Reserved.
47 
48 Licensed under the Apache License 2.0 (the "License").  You may not use
49 this file except in compliance with the License.  You can obtain a copy
50 in the file LICENSE in the source distribution or at
51 L<https://www.openssl.org/source/license.html>.
52 
53 =cut
54