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Tue, 08 Aug 2023 03:21:26 -0700 (PDT) Date: Tue, 8 Aug 2023 12:17:25 +0200 Mime-Version: 1.0 X-Mailer: git-send-email 2.41.0.640.ga95def55d0-goog Message-ID: <20230808102049.465864-1-elver@google.com> Subject: [PATCH v3 1/3] compiler_types: Introduce the Clang __preserve_most function attribute From: Marco Elver To: elver@google.com, Andrew Morton , Kees Cook Cc: Guenter Roeck , Peter Zijlstra , Mark Rutland , Steven Rostedt , Marc Zyngier , Oliver Upton , James Morse , Suzuki K Poulose , Zenghui Yu , Catalin Marinas , Will Deacon , Nathan Chancellor , Nick Desaulniers , Tom Rix , Miguel Ojeda , Sami Tolvanen , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, llvm@lists.linux.dev, Dmitry Vyukov , Alexander Potapenko , kasan-dev@googlegroups.com, linux-toolchains@vger.kernel.org X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20230808_032129_816007_5E8A4134 X-CRM114-Status: GOOD ( 19.43 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org [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). Signed-off-by: Marco Elver Reviewed-by: Miguel Ojeda Acked-by: Steven Rostedt (Google) Acked-by: Mark Rutland --- v3: * Quote more from LLVM documentation about which registers are callee/caller with preserve_most. * Code comment to restrict use where tracing is meant to be disabled. v2: * Imply notrace, to avoid any conflicts with tracing which is inserted on function entry. See added comments. --- include/linux/compiler_types.h | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 insertions(+) diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h index 547ea1ff806e..c88488715a39 100644 --- a/include/linux/compiler_types.h +++ b/include/linux/compiler_types.h @@ -106,6 +106,34 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { } #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 */ /*