| =pod |
| |
| =head1 NAME |
| |
| OPENSSL_ia32cap - finding the IA-32 processor capabilities |
| |
| =head1 SYNOPSIS |
| |
| unsigned long *OPENSSL_ia32cap_loc(void); |
| #define OPENSSL_ia32cap (*(OPENSSL_ia32cap_loc())) |
| |
| =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 |
| register after executing CPUID instruction with EAX=1 input value (see |
| Intel Application Note #241618). Naturally it's meaningful on IA-32[E] |
| 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 six bits are |
| significant, namely: |
| |
| 1. bit #28 denoting Hyperthreading, which is used to distiguish |
| cores with shared cache; |
| 2. bit #26 denoting SSE2 support; |
| 3. bit #25 denoting SSE support; |
| 4. bit #23 denoting MMX support; |
| 5. bit #20, reserved by Intel, is used to choose between RC4 code |
| pathes; |
| 6. bit #4 denoting presence of Time-Stamp Counter. |
| |
| 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. |
| |
| =cut |