=head1 NAME
-OPENSSL_ia32cap - finding the IA-32 processor capabilities
+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()))
+ unsigned int *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
-register after executing CPUID instruction with EAX=1 input value (see
-Intel Application Note #241618). Naturally it's meaningful on IA-32[E]
-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 five bits are
-significant, namely bit #28 denoting Hyperthreading, which is used to
-distinguish Intel P4 core, bit #26 denoting SSE2 support, bit #25
-denoting SSE support, bit #23 denoting MMX support, and bit #4 denoting
-presence of Time-Stamp Counter. Clearing bit #26 at run-time for
-example disables high-performance SSE2 code present in the crypto
-library. You might have to do this if target OpenSSL application is
-executed on SSE2 capable CPU, but under control of OS which does not
-support SSE2 extentions. Even though you can manipulate the value
+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 #54 denoting availability of MOVBE instruction;
+
+=item bit #57 denoting AES-NI instruction set extension;
+
+=item bit #58, XSAVE bit, lack of which in combination with MOVBE is used
+to identify Atom Silvermont core;
+
+=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, 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. 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. 'env OPENSSL_ia32cap=0x12800010 apps/openssl', to
-achieve same effect without modifying the application source code.
-Alternatively you can reconfigure the toolkit with no-sse2 option and
-recompile.
+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 capability 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. MULX
+and RORX;
+
+=item bit #64+16 denoting availability of AVX512F extension;
+
+=item bit #64+18 denoting availability of RDSEED instruction;
+
+=item bit #64+19 denoting availability of ADCX and ADOX instructions;
+
+=item bit #64+29 denoting availability of SHA extension;
+
+=item bit #64+30 denoting availability of AVX512BW extension;
+
+=item bit #64+31 denoting availability of AVX512VL extension;
+
+=back
+
+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.
+
+It should be noted that whether or not some of the most "fancy"
+extension code paths are actually assembled depends on current assembler
+version. Base minimum of AES-NI/PCLMULQDQ, SSSE3 and SHA extension code
+paths are always assembled. Besides that, minimum assembler version
+requirements are summarized in below table:
+
+ Extension | GNU as | nasm | llvm
+ ------------+--------+--------+--------
+ AVX | 2.19 | 2.09 | 3.0
+ AVX2 | 2.22 | 2.10 | 3.1
+ AVX512 | 2.25 | 2.11.8 | 3.6
+
+=head1 COPYRIGHT
+
+Copyright 2004-2016 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the OpenSSL license (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
+L<https://www.openssl.org/source/license.html>.
=cut
projects / openssl.git / blobdiff