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doc/crypto/OPENSSL_ia32cap.pod update [from HEAD].

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Andy Polyakov 2007-11-11 20:10:09 +00:00
parent 2ea3cd8abc
commit a1cc568288
1 changed files with 22 additions and 14 deletions
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doc/crypto/OPENSSL_ia32cap.pod
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@ -17,19 +17,27 @@ 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 three bits are
significant, namely bit #28 denoting Hyperthreading, which is used to
distinguish Intel P4 core, bit #26 denoting SSE2 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
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=0x10 apps/openssl', to achieve
same effect without modifying the application source code.
Alternatively you can reconfigure the toolkit with no-sse2 option and
recompile.
crypto library behaviour. For the moment of this writing six bits are
significant, namely:
1. bit #28 denoting Hyperthreading, which is used to distiguish
cores with shared cache;
2. bit #26 denoting SSE2 support;
3. bit #25 denoting SSE support;
4. bit #23 denoting MMX support;
5. bit #20, reserved by Intel, is used to choose between RC4 code
pathes;
6. bit #4 denoting presence of Time-Stamp Counter.
For example, clearing bit #26 at run-time 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 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. on Intel P4 processor 'env
OPENSSL_ia32cap=0x12900010 apps/openssl', to achieve same effect
without modifying the application source code. Alternatively you can
reconfigure the toolkit with no-sse2 option and recompile.
=cut