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37.4 Hardware Capability Tunables

Tunable namespace: glibc.tune

Behavior of the GNU C Library can be tuned to assume specific hardware capabilities by setting the following tunables in the tune namespace:

Tunable: glibc.tune.hwcap_mask

This tunable supersedes the LD_HWCAP_MASK environment variable and is identical in features.

The AT_HWCAP key in the Auxiliary Vector specifies instruction set extensions available in the processor at runtime for some architectures. The glibc.tune.hwcap_mask tunable allows the user to mask out those capabilities at runtime, thus disabling use of those extensions.

Tunable: glibc.tune.hwcaps

The glibc.tune.hwcaps=-xxx,yyy,-zzz... tunable allows the user to enable CPU/ARCH feature yyy, disable CPU/ARCH feature xxx and zzz where the feature name is case-sensitive and has to match the ones in sysdeps/x86/cpu-features.h.

This tunable is specific to i386 and x86-64.

Tunable: glibc.tune.cached_memopt

The glibc.tune.cached_memopt=[0|1] tunable allows the user to enable optimizations recommended for cacheable memory. If set to 1, the GNU C Library assumes that the process memory image consists of cacheable (non-device) memory only. The default, 0, indicates that the process may use device memory.

This tunable is specific to powerpc, powerpc64 and powerpc64le.

Tunable: glibc.tune.cpu

The glibc.tune.cpu=xxx tunable allows the user to tell the GNU C Library to assume that the CPU is xxx where xxx may have one of these values: generic, falkor, thunderxt88, thunderx2t99, thunderx2t99p1.

This tunable is specific to aarch64.

Tunable: glibc.tune.x86_data_cache_size

The glibc.tune.x86_data_cache_size tunable allows the user to set data cache size in bytes for use in memory and string routines.

This tunable is specific to i386 and x86-64.

Tunable: glibc.tune.x86_shared_cache_size

The glibc.tune.x86_shared_cache_size tunable allows the user to set shared cache size in bytes for use in memory and string routines.

Tunable: glibc.tune.x86_non_temporal_threshold

The glibc.tune.x86_non_temporal_threshold tunable allows the user to set threshold in bytes for non temporal store.

This tunable is specific to i386 and x86-64.

Tunable: glibc.tune.x86_ibt

The glibc.tune.x86_ibt tunable allows the user to control how indirect branch tracking (IBT) should be enabled. Accepted values are on, off, and permissive. on always turns on IBT regardless of whether IBT is enabled in the executable and its dependent shared libraries. off always turns off IBT regardless of whether IBT is enabled in the executable and its dependent shared libraries. permissive is the same as the default which disables IBT on non-CET executables and shared libraries.

This tunable is specific to i386 and x86-64.

Tunable: glibc.tune.x86_shstk

The glibc.tune.x86_shstk tunable allows the user to control how the shadow stack (SHSTK) should be enabled. Accepted values are on, off, and permissive. on always turns on SHSTK regardless of whether SHSTK is enabled in the executable and its dependent shared libraries. off always turns off SHSTK regardless of whether SHSTK is enabled in the executable and its dependent shared libraries. permissive changes how dlopen works on non-CET shared libraries. By default, when SHSTK is enabled, dlopening a non-CET shared library returns an error. With permissive, it turns off SHSTK instead.

This tunable is specific to i386 and x86-64.


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