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Heiko writes: "The recent large kernel Rust thread where Linus commented about that structures may be returned in registers [1] made me again aware that this is not true for s390 where the ABI defines that structures are returned in a return value buffer allocated by the caller. This was also mentioned by Alexander Gordeev a couple of weeks ago. In theory the -freg-struct-return compiler flag would allow to return small structures in registers, however that has not been implemented for s390. Juergen Christ did an experimental gcc implementation which shows the benefit of such a change (bloat-o-meter): add/remove: 3/2 grow/shrink: 12/441 up/down: 740/-7182 (-6442) This result is not very impressive, and doesn't seem to justify a new ABI for the kernel. However there is still the existing STRICT_MM_TYPECHECKS which can be used to change some mm types from structures to simple scalar types. Changing the mm types results in: add/remove: 2/8 grow/shrink: 25/116 up/down: 3902/-6204 (-2302) Which is already a third of the possible savings which would be the result of the described ABI change. Therefore add support for a configurable STRICT_MM_TYPECHECKS which allows to generate better code, but also allows to have type checking for debug builds." [1] https://lore.kernel.org/all/CAHk-=wgb1g9VVHRaAnJjrfRFWAOVT2ouNOMqt0js8h3D6zvHDw@mail.gmail.com/ * strict-mm-typechecks-support: s390/mm: Add configurable STRICT_MM_TYPECHECKS s390/mm: Convert pgste_val() into function s390/mm: Convert pgprot_val() into function s390/mm: Use pgprot_val() instead of open coding Signed-off-by: Vasily Gorbik <gor@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 reStructuredText 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.