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The compiler emitted hook used for ftrace consists of a PUSH {LR} to
preserve the link register, followed by a branch-and-link (BL) to
__gnu_mount_nc. Dynamic ftrace patches away the latter to turn the
combined sequence into a NOP, using a POP {LR} instruction.
This is not necessary, since the link register does not get clobbered in
this case, and simply adding #4 to the stack pointer is sufficient, and
avoids a memory access that may take a few cycles to resolve depending
on the micro-architecture.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
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.