The patch titled
Subject: compiler_types: introduce the Clang __preserve_most function attribute
has been added to the -mm mm-nonmm-unstable branch. Its filename is
compiler_types-introduce-the-clang-__preserve_most-function-attribute.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/compiler_types-introduce-the-clang-__preserve_most-function-attribute.patch
This patch will later appear in the mm-nonmm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Marco Elver <elver@xxxxxxxxxx>
Subject: compiler_types: introduce the Clang __preserve_most function attribute
Date: Tue, 8 Aug 2023 12:17:25 +0200
[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.
The attribute may be supported by a future GCC version (see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110899).
Link: https://lkml.kernel.org/r/20230808102049.465864-1-elver@xxxxxxxxxx
Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Reviewed-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
Acked-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Acked-by: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Alexander Potapenko <glider@xxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Guenter Roeck <linux@xxxxxxxxxxxx>
Cc: James Morse <james.morse@xxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Marc Zyngier <maz@xxxxxxxxxx>
Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: Oliver Upton <oliver.upton@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
Cc: Tom Rix <trix@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Zenghui Yu <yuzenghui@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/compiler_types.h | 28 ++++++++++++++++++++++++++++
1 file changed, 28 insertions(+)
--- a/include/linux/compiler_types.h~compiler_types-introduce-the-clang-__preserve_most-function-attribute
+++ a/include/linux/compiler_types.h
@@ -106,6 +106,34 @@ static inline void __chk_io_ptr(const vo
#define __cold
#endif
+/*
+ * On x86-64 and arm64 targets, __preserve_most changes the calling convention
+ * of a function 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 uses a different set of caller- and callee-
+ * saved registers.
+ *
+ * The purpose is to alleviates the burden of saving and recovering a large
+ * register set before and after the call in the caller. This is beneficial for
+ * rarely taken slow paths, such as error-reporting functions that may be called
+ * from hot paths.
+ *
+ * Note: This may conflict with instrumentation inserted on function entry which
+ * does not use __preserve_most or equivalent convention (if in assembly). Since
+ * function tracing assumes the normal C calling convention, where the attribute
+ * is supported, __preserve_most implies notrace. It is recommended to restrict
+ * use of the attribute to functions that should or already disable tracing.
+ *
+ * Optional: not supported by gcc.
+ *
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#preserve-most
+ */
+#if __has_attribute(__preserve_most__)
+# define __preserve_most notrace __attribute__((__preserve_most__))
+#else
+# define __preserve_most
+#endif
+
/* Builtins */
/*
_
Patches currently in -mm which might be from elver@xxxxxxxxxx are
compiler_types-introduce-the-clang-__preserve_most-function-attribute.patch
list_debug-introduce-inline-wrappers-for-debug-checks.patch
list_debug-introduce-config_debug_list_minimal.patch
