OPENSSL_IA32CAP(3SSL)

📦
version 4.0.3 · 13 versions
, in ubuntu stonking

💖 NAME 💖

OPENSSL_ia32cap — the x86[_64] processor capabilities vector, aka the lil list of "what can ur CPU actually do tho" 🧚

✨ SYNOPSIS ✨

env OPENSSL_ia32cap=... <application>

🦋 DESCRIPTION 🦋

okay bestie, sit down, hydrate, this one's a bop. OpenSSL supports a whole range of x86[_64] instruction set extensions and features — think of them as the CPU's hidden talents. 💅 Those extensions get tracked by individual bits, or lil groups of bits, stored internally as ten 32-bit capability vectors, and for convenience they're shown logically below as five 64-bit vectors. That logical vector (LV) thing? It exists purely to make defining the OPENSSL_ia32cap environment variable less of a nightmare, and honestly? iconic. 👑

When the toolkit initializes, those capability vectors get populated through successive executions of the CPUID instruction, and then whatever capability bit modifications you put in the OPENSSL_ia32cap environment variable get applied on top. Once initialization is done-done, the populated vectors are used to pick between different code paths, so everything runs optimally across a huge range of x86[_64] based processors. slay. ⚡

Wanna know even more CPUID lore? Go read the Intel(R) Architecture Instruction Set Extensions Programming Reference, and the AMD64 Architecture Programmer's Manual (Volume 3). iykyk 📚

🌸 Notable Capability Bits for LV0 🌸

These are the notable capability bits from logical vector 0 (LV0), and they come from the following execution of CPUID.(EAX=01H).EDX and CPUID.(EAX=01H).ECX:

  • bit #0+4 — denoting presence of Time-Stamp Counter
  • bit #0+19 — denoting availability of CLFLUSH instruction
  • bit #0+20 — reserved by Intel, is used to choose among RC4 code paths
  • bit #0+23 — denoting MMX support
  • bit #0+24 — FXSR bit, denoting availability of XMM registers
  • bit #0+25 — denoting SSE support
  • bit #0+26 — denoting SSE2 support
  • bit #0+28 — denoting Hyperthreading, which is used to distinguish cores with shared cache
  • bit #0+30 — reserved by Intel, denotes specifically Intel CPUs
  • bit #0+33 — denoting availability of PCLMULQDQ instruction
  • bit #0+41 — denoting SSSE3, Supplemental SSE3, support
  • bit #0+43 — denoting AMD XOP support (forced to zero on non-AMD CPUs)
  • bit #0+54 — denoting availability of MOVBE instruction
  • bit #0+57 — denoting AES-NI instruction set extension
  • bit #0+58 — XSAVE bit, lack of which in combination with MOVBE is used to identify Atom Silvermont core
  • bit #0+59 — OSXSAVE bit, denoting availability of YMM registers
  • bit #0+60 — denoting AVX extension
  • bit #0+62 — denoting availability of RDRAND instruction

🎀 Notable Capability Bits for LV1 🎀

The notable capability bits from logical vector 1 (LV1), coming from the following execution of CPUID.(EAX=07H,ECX=0H).EBX and CPUID.(EAX=07H,ECX=0H).ECX:

  • 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 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

💜 Notable Capability Bits for LV2 💜

The notable capability bits from logical vector 2 (LV2), coming from the following execution of CPUID.(EAX=07H,ECX=0H).EDX and CPUID.(EAX=07H,ECX=1H).EAX:

  • bit #128+15 — denoting availability of Hybrid CPU
  • bit #128+29 — denoting support for IA32_ARCH_CAPABILITIES MSR
  • bit #128+32 — denoting availability of SHA512 extension
  • bit #128+33 — denoting availability of SM3 extension
  • bit #128+34 — denoting availability of SM4 extension
  • bit #128+55 — denoting availability of AVX-IFMA extension

🌈 Notable Capability Bits for LV3 🌈

The notable capability bits from logical vector 3 (LV3), coming from the following execution of CPUID.(EAX=07H,ECX=1H).EDX and CPUID.(EAX=07H,ECX=1H).EBX:

  • bit #192+19 — denoting availability of AVX10 Converged Vector ISA extension
  • bit #192+21 — denoting availability of APX_F extension

⭐ Notable Capability Bits for LV4 ⭐

The notable capability bits from logical vector 4 (LV4), coming from the following execution of CPUID.(EAX=07H,ECX=1H).ECX and CPUID.(EAX=24H,ECX=0H).EBX:

  • bits #256+32+[0:7] — denoting AVX10 Converged Vector ISA Version (8 bits)
  • bit #256+48 — denoting AVX10 XMM support
  • bit #256+49 — denoting AVX10 YMM support
  • bit #256+50 — denoting AVX10 ZMM support

💅 OPENSSL_ia32cap environment variable 💅

The OPENSSL_ia32cap environment variable is literally the mechanism that lets you override the default capability vector values at library initialization time. It's made up of a series of 64-bit numbers, one per logical vector (LV) from above. Each value is delimited by a ':'. Decimal/Octal/Hexadecimal values representations are supported. (yes, all three, we love options 🫶)

"env OPENSSL_ia32cap=LV0:LV1:LV2:LV3:LV4"

Used in this form, each non-null logical vector will *overwrite* the entire corresponding capability vector pair with the provided value. And to keep compatibility with the behaviour of the original OPENSSL_ia32cap environment variable <env OPENSSL_ia32cap=LV0:LV1>, the next capability vector pairs will be set to zero.

To illustrate, the following will zero all capability bits in logical vectors 1 and further (disable all post-AVX extensions):

"env OPENSSL_ia32cap=:0"

The following will zero all capability bits in logical vectors 2 and further:

"env OPENSSL_ia32cap=::0"

The following will zero all capability bits only in logical vector 1:

"env OPENSSL_ia32cap=:0::::"

A more likely usage scenario would be to disable specific instruction set extensions. The '~' character is used to specify a bit mask of the extensions to be disabled for a particular logical vector. 🎯

To illustrate, the following will disable AVX2 code paths and further extensions:

"env OPENSSL_ia32cap=:~0x20000000000"

The following will disable AESNI (LV0 bit 57) and VAES (LV1 bit 41) extensions and therefore any code paths using those extensions but leave the rest of the logical vectors unchanged:

"env OPENSSL_ia32cap=~0x200000000000000:~0x20000000000:~0x0:~0x0:~0x0"

🔥 NOTES 🔥

not every capability bit is copied from CPUID output verbatim, bestie. 💀

An example of this is the somewhat less intuitive clearing of LV0 bit #28, or ~0x10000000 in the "environment variable" terms. It has been adjusted to reflect whether or not the data cache is actually shared between logical cores. Which in turn affects the decision on whether or not expensive countermeasures against cache-timing attacks are applied, most notably in AES assembler module. so yeah, that bit is doing heavy lifting. 😤

💀 RETURN VALUES 💀

Not available. (nope, nothing, nada 🙈)

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>.

okay that's the whole thing bestie, you ate that 💖 this was OpenSSL 4.0.3, dated 2026-09-29, and it's the OPENSSL_IA32CAP(3SSL) page — go forth and vibe check ur CPU 🌟

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