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s390 has instructions to support 128 bit arithmetics, e.g. a 64 bit multiply instruction with a 128 bit result. Also 128 bit integer artithmetics are already used in s390 specific architecture code (see e.g. read_persistent_clock64()). Therefore select ARCH_SUPPORTS_INT128. However limit this to clang for now, since gcc generates inefficient code, which may lead to stack overflows, when compiling lib/crypto/curve25519-hacl64.c which depends on ARCH_SUPPORTS_INT128. The gcc generated functions have 6kb stack frames, compared to only 1kb of the code generated with clang. If the kernel is compiled with -Os library calls for __ashlti3(), __ashrti3(), and __lshrti3() may be generated. Similar to arm64 and riscv provide assembler implementations for these functions. Acked-by: Alexander Gordeev <agordeev@linux.ibm.com> Signed-off-by: Heiko Carstens <hca@linux.ibm.com> Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.