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[1]: "On X86-64 and AArch64 targets, this attribute changes the calling convention of a function. The preserve_most calling convention attempts to make the code in the caller as unintrusive as possible. This convention behaves identically to the C calling convention on how arguments and return values are passed, but it uses a different set of caller/callee-saved registers. This alleviates the burden of saving and recovering a large register set before and after the call in the caller. If the arguments are passed in callee-saved registers, then they will be preserved by the callee across the call. This doesn't apply for values returned in callee-saved registers. * On X86-64 the callee preserves all general purpose registers, except for R11. R11 can be used as a scratch register. Floating-point registers (XMMs/YMMs) are not preserved and need to be saved by the caller. * On AArch64 the callee preserve all general purpose registers, except x0-X8 and X16-X18." [1] https://clang.llvm.org/docs/AttributeReference.html#preserve-most Introduce the attribute to compiler_types.h as __preserve_most. Use of this attribute results in better code generation for calls to very rarely called functions, such as error-reporting functions, or rarely executed slow paths. Beware that the attribute conflicts with instrumentation calls inserted on function entry which do not use __preserve_most themselves. Notably, function tracing which assumes the normal C calling convention for the given architecture. Where the attribute is supported, __preserve_most will imply notrace. It is recommended to restrict use of the attribute to functions that should or already disable tracing. Note: The additional preprocessor check against architecture should not be necessary if __has_attribute() only returns true where supported; also see https://github.com/ClangBuiltLinux/linux/issues/1908. But until __has_attribute() does the right thing, we also guard by known-supported architectures to avoid build warnings on other architectures. The attribute may be supported by a future GCC version (see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110899). Signed-off-by: Marco Elver <elver@google.com> Reviewed-by: Miguel Ojeda <ojeda@kernel.org> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> Acked-by: "Steven Rostedt (Google)" <rostedt@goodmis.org> Acked-by: Mark Rutland <mark.rutland@arm.com> Link: https://lore.kernel.org/r/20230811151847.1594958-1-elver@google.com Signed-off-by: Kees Cook <keescook@chromium.org> |
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| README | ||
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.