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OPENSSL_IA32CAP(3ossl) OpenSSL OPENSSL_IA32CAP(3ossl)
NAME
OPENSSL_ia32cap - the x86[_64] processor capabilities vector
SYNOPSIS
env OPENSSL_ia32cap=... <application>
DESCRIPTION
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
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.
RETURN VALUES
Not available.
COPYRIGHT
Copyright 2004-2021 The OpenSSL Project Authors. All Rights Reserved.
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>.
3.0.11 2023-09-19 OPENSSL_IA32CAP(3ossl)