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The function takes the register number, finds the corresponding field of pt_regs for registers that are saved there or does the unwind for the registers that end up spilled on the kernel stack. Then it reads from or writes to the resulting location. Unfortunately, finding the required pt_regs field is done by rather horrible switch. It's microoptimized in all the wrong places - it even uses the knowledge that fields for r8..r11 follow each other in pt_regs layout, while r12..r13 are not adjacent to those, etc. All of that is to encode the mapping from register numbers to offsets + the information that r4..r7 are not to be found in pt_regs. It's deeply in nasal demon territory, at that - the games it plays with pointer arithmetics on addresses of structure members are undefined behaviour. Valid C ends up with better code in this case: just initialize a constant array with offsets of relevant pt_regs fields and we don't need that switch anymore. Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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.