source: vbox/trunk/src/libs/openssl-1.1.1h/doc/man3/OPENSSL_ia32cap.pod@ 86719

=pod

=head1 NAME

OPENSSL_ia32cap - the x86[_64] processor capabilities vector

=head1 SYNOPSIS

 env OPENSSL_ia32cap=... <application>

=head1 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:

=over 4

=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. 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:

=over 4

=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+21 denoting availability of VPMADD52[LH]UQ instructions,
aka AVX512IFMA extension;

=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;

=item bit #64+41 denoting availability of VAES extension;

=item bit #64+42 denoting availability of VPCLMULQDQ 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. Apart from 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
   ADCX/ADOX   | 2.23   | 2.10   | 3.3
   AVX512      | 2.25   | 2.11.8 | see NOTES
   AVX512IFMA  | 2.26   | 2.11.8 | see NOTES
   VAES        | 2.30   | 2.13.3 |

=head1 NOTES

Even though AVX512 support was implemented in llvm 3.6, compilation of
assembly modules apparently requires explicit -march flag. But then
compiler generates processor-specific code, which in turn contradicts
the mere idea of run-time switch execution facilitated by the variable
in question. Till the limitation is lifted, it's possible to work around
the problem by making build procedure use following script:

   #!/bin/sh
   exec clang -no-integrated-as "$@"

instead of real clang. In which case it doesn't matter which clang
version is used, as it is GNU assembler version that will be checked.

=head1 RETURN VALUES

Not available.

=head1 COPYRIGHT

Copyright 2004-2020 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>.

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