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OPENSSL_ia32cap − the x86[_64] processor capabilities vector
env OPENSSL_ia32cap=... <application>
OpenSSL
supports a range of x86[_64] instruction set extensions.
These extensions are denoted by individual bits in
capability vector returned by processor in
EDX:ECX register pair after executing
CPUID instruction with EAX=1 input value (see
Intel Application Note #241618). This vector is copied to
memory upon toolkit initialization and used to choose
between different code paths to provide optimal performance
across wide range of processors. For the moment of this
writing following bits are significant:
bit #4 denoting presence of Time-Stamp Counter.
bit #19 denoting availability of CLFLUSH
instruction;
bit #20, reserved by Intel, is used to choose among
RC4 code paths;
bit #23 denoting MMX support;
bit #24, FXSR bit, denoting availability of
XMM registers;
bit #25 denoting SSE support;
bit #26 denoting SSE2 support;
bit #28 denoting Hyperthreading, which is used to
distinguish cores
with shared cache;
bit #30, reserved by Intel, denotes specifically Intel CPUs;
bit #33 denoting availability of PCLMULQDQ
instruction;
bit #41 denoting SSSE3, Supplemental
SSE3, support;
bit #43 denoting AMD XOP support (forced to
zero on non-AMD CPUs);
bit #54 denoting availability of MOVBE
instruction;
bit #57 denoting AES-NI instruction set extension;
bit #58, XSAVE bit, lack of which in
combination with MOVBE is used to
identify Atom Silvermont core;
bit #59, OSXSAVE bit, denoting availability
of YMM registers;
bit #60 denoting AVX extension;
bit #62 denoting availability of RDRAND
instruction;
For example, in 32−bit application context 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. Historically address of the capability vector copy was exposed to application through OPENSSL_ia32cap_loc(), but not anymore. Now the only way to affect the capability detection is to set OPENSSL_ia32cap environment variable prior target application start. To give a specific example, on Intel P4 processor "env OPENSSL_ia32cap=0x16980010 apps/openssl", or better yet "env OPENSSL_ia32cap=~0x1000000 apps/openssl" would achieve the desired effect. Alternatively you can reconfigure the toolkit with no−sse2 option and recompile.
Less intuitive is clearing bit #28, or ~0x10000000 in the "environment variable" terms. 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 capability
vector is further extended with EBX value
returned by CPUID with EAX=7 and ECX=0 as
input. Following bits are significant:
bit #64+3 denoting availability of BMI1
instructions, e.g. ANDN ;
bit #64+5 denoting availability of AVX2
instructions;
bit #64+8 denoting availability of BMI2
instructions, e.g. MULX and
RORX ;
bit #64+16 denoting availability of AVX512F
extension;
bit #64+17 denoting availability of AVX512DQ
extension;
bit #64+18 denoting availability of RDSEED
instruction;
bit #64+19 denoting availability of ADCX and
ADOX instructions;
bit #64+21 denoting availability of VPMADD52[
LH ]UQ instructions, aka
AVX512IFMA extension;
bit #64+29 denoting availability of SHA
extension;
bit #64+30 denoting availability of AVX512BW
extension;
bit #64+31 denoting availability of AVX512VL
extension;
bit #64+41 denoting availability of VAES
extension;
bit #64+42 denoting availability of
VPCLMULQDQ extension;
To control this extended capability word use ":" as delimiter when setting up OPENSSL_ia32cap environment variable. For example assigning ":~0x20" would disable AVX2 code paths, and ":0" − all post-AVX extensions.
Not available.
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Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.