S390: Influence hwcaps/stfle via GLIBC_TUNABLES.

This patch enables the option to influence hwcaps and stfle bits used
by the s390 specific ifunc-resolvers.  The currently x86-specific
tunable glibc.cpu.hwcaps is also used on s390x to achieve the task. In
addition the user can also set a CPU arch-level like z13 instead of
single HWCAP and STFLE features.

Note that the tunable only handles the features which are really used
in the IFUNC-resolvers.  All others are ignored as the values are only
used inside glibc.  Thus we can influence:
- HWCAP_S390_VXRS (z13)
- HWCAP_S390_VXRS_EXT (z14)
- HWCAP_S390_VXRS_EXT2 (z15)
- STFLE_MIE3 (z15)

The influenced hwcap/stfle-bits are stored in the s390-specific
cpu_features struct which also contains reserved fields for future
usage.

The ifunc-resolvers and users of stfle bits are adjusted to use the
information from cpu_features struct.

On 31bit, the ELF_MACHINE_IRELATIVE macro is now also defined.
Otherwise the new ifunc-resolvers segfaults as they depend on
the not yet processed_rtld_global_ro@GLIBC_PRIVATE relocation.

1 files changed, 5 insertions, 1 deletions

diff --git a/manual/tunables.texi b/manual/tunables.texi
index ffd50a3..c2630b8 100644
--- a/manual/tunables.texi
+++ b/manual/tunables.texi
@@ -486,7 +486,11 @@ enable CPU/ARCH feature @code{yyy}, disable CPU/ARCH feature @code{xxx}
 and @code{zzz} where the feature name is case-sensitive and has to match
 the ones in @code{sysdeps/x86/include/cpu-features.h}.
 
-This tunable is specific to i386 and x86-64.
+On s390x, the supported HWCAP and STFLE features can be found in
+@code{sysdeps/s390/cpu-features.c}.  In addition the user can also set
+a CPU arch-level like @code{z13} instead of single HWCAP and STFLE features.
+
+This tunable is specific to i386, x86-64 and s390x.
 @end deftp
 
 @deftp Tunable glibc.cpu.cached_memopt