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-=pod
-
-=head1 NAME
-
-OPENSSL_ia32cap, OPENSSL_ia32cap_loc - the IA-32 processor capabilities vector
-
-=head1 SYNOPSIS
-
- unsigned long *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 following bits are significant:
-
-=over
-
-=item bit #4 denoting presence of Time-Stamp Counter.
-
-=item bit #19 denoting availability of CLFLUSH instruction;
-
-=item bit #20, reserved by Intel, is used to choose among RC4 code paths;
-
-=item bit #23 denoting MMX support;
-
-=item bit #24, FXSR bit, denoting availability of XMM registers;
-
-=item bit #25 denoting SSE support;
-
-=item bit #26 denoting SSE2 support;
-
-=item bit #28 denoting Hyperthreading, which is used to distinguish
-cores with shared cache;
-
-=item bit #30, reserved by Intel, denotes specifically Intel CPUs;
-
-=item bit #33 denoting availability of PCLMULQDQ instruction;
-
-=item bit #41 denoting SSSE3, Supplemental SSE3, support;
-
-=item bit #43 denoting AMD XOP support (forced to zero on non-AMD CPUs);
-
-=item bit #57 denoting AES-NI instruction set extension;
-
-=item bit #59, OSXSAVE bit, denoting availability of YMM registers;
-
-=item bit #60 denoting AVX extension;
-
-=item bit #62 denoting availability of RDRAND instruction;
-
-=back
-
-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:
-
-=over
-
-=item bit #64+3 denoting availability of BMI1 instructions, e.g. ANDN;
-
-=item bit #64+5 denoting availability of AVX2 instructions;
-
-=item bit #64+8 denoting availability of BMI2 instructions, e.g. MUXL
-and RORX;
-
-=item bit #64+18 denoting availability of RDSEED instruction;
-
-=item bit #64+19 denoting availability of ADCX and ADOX instructions;
-
-=back