OPENSSL_ia32cap, OPENSSL_ia32cap_loc - the IA-32 processor capabilities vector
unsigned long *OPENSSL_ia32cap_loc(void); #define OPENSSL_ia32cap ((OPENSSL_ia32cap_loc()))
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 following bits are significant:
For example, clearing bit #26 at run-time disables high-performance SSE2 code present in the crypto library, while clearing bit #24 disables SSE2 code operating on 128-bit XMM register bank. You might have to do the latter if target OpenSSL application is executed on SSE2 capable CPU, but under control of OS that does not enable XMM registers. 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=0x16980010 apps/openssl', or better yet 'env OPENSSL_ia32cap=~0x1000000 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 intuitive 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.
The vector is further extended with EBX value returned by CPUID with EAX=7 and ECX=0 as input. Following bits are significant: