openssl/doc/crypto/OPENSSL_ia32cap.pod

=pod

=head1 NAME

OPENSSL_ia32cap - finding the IA-32 processor capabilities

=head1 SYNOPSIS

 unsigned int *OPENSSL_ia32cap_loc(void);
 #define OPENSSL_ia32cap ((OPENSSL_ia32cap_loc())[0])

=head1 DESCRIPTION

Value returned by OPENSSL_ia32cap_loc() is address of a variable
containing IA-32 processor capabilities bit vector as it appears in
EDX:ECX register pair after executing CPUID instruction with EAX=1
input value (see Intel Application Note #241618). Naturally it's
meaningful on x86 and x86_64 platforms only. The variable is normally
set up automatically upon toolkit initialization, but can be
manipulated afterwards to modify crypto library behaviour. For the
moment of this writing seven bits are significant, namely:

1. bit #4 denoting presence of Time-Stamp Counter.
2. bit #20, reserved by Intel, is used to choose between RC4 code
   paths;
3. bit #23 denoting MMX support;
4. bit #25 denoting SSE support;
5. bit #26 denoting SSE2 support;
6. bit #28 denoting Hyperthreading, which is used to distiguish
   cores with shared cache;
7. bit #57 denoting Intel AES instruction set extension;

For example, clearing bit #26 at run-time disables high-performance
SSE2 code present in the crypto library. You might have to do this if
target OpenSSL application is executed on SSE2 capable CPU, but under
control of OS which does not support SSE2 extentions. Even though you
can manipulate the value programmatically, you most likely will find it
more appropriate to set up an environment variable with the same name
prior starting target application, e.g. on Intel P4 processor 'env
OPENSSL_ia32cap=0x12900010 apps/openssl', to achieve same effect
without modifying the application source code. Alternatively you can
reconfigure the toolkit with no-sse2 option and recompile.

Less intuituve is clearing bit #28. The truth is that it's not copied
from CPUID output verbatim, but is adjusted to reflect whether or not
the data cache is actually shared between logical cores. This in turn
affects the decision on whether or not expensive countermeasures
against cache-timing attacks are applied, most notably in AES assembler
module.
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