From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-yw1-f202.google.com (mail-yw1-f202.google.com [209.85.128.202]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id F187663B6 for ; Fri, 11 Aug 2023 15:19:58 +0000 (UTC) Received: by mail-yw1-f202.google.com with SMTP id 00721157ae682-5897d05e878so25195687b3.3 for ; Fri, 11 Aug 2023 08:19:58 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20221208; t=1691767198; x=1692371998; h=cc:to:from:subject:message-id:mime-version:date:from:to:cc:subject :date:message-id:reply-to; bh=1vaThr58cHhzt3vCJBdqBIs2Yl/3Z2jvMp4OgQvCHVc=; b=5xqJ+9vOp2nrO/mxnyBJ/b/1HWLsgBT1IHmI50RXMG+v00b5fp1vs2Rwd8BJ+9On+a Q4jl5SsKlofCJHbO3RpNFGWeldC2dgdOf7WiBs7B3msfJcvntjhupdIzPNHiuqPJVz+z R/HdlzmM4cQEmJpgDlmTthNmGG/KyvuF0gdplPRvM4udG74+d0udlvizD0YfOPv0Opv2 3tPbceoZMBOxCBuYZR9Iewa4BKkdIHfl4kSzyUUw34WMnBaidreMjrhu73PoE0V1Y3HY Lo9Pzzu3F/0G/X2jav91I9Oukq8tP4eQTMYHi0nGsMKpFT4HL5VWbyKeiy/SS6PBE/G+ bSHA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1691767198; x=1692371998; h=cc:to:from:subject:message-id:mime-version:date:x-gm-message-state :from:to:cc:subject:date:message-id:reply-to; bh=1vaThr58cHhzt3vCJBdqBIs2Yl/3Z2jvMp4OgQvCHVc=; b=NU8ysSAxOqrHor2E1e7wwEZLQJ5snars8j4dHOD8GFEI7ry7dPkUfAVa/Es4Bqa7Nt TGsEYWI8SSKJ1X+gHFsoj/QBsApdYCorIlgDdzqap3etK6YicuTeN5em8WCPfy5jENSH 77WnQKIS0qd5Xdb8ufso+jEvVW6gtp1TGMARJBotzFmJK17w39AsOTsF2uJJKgXJDA07 l0JcEzEp47vqf3hiOJHsnbO3twFH+tOCb5K10Z90Z/xm88+PY+83WXrMPwcgGt6r7tJM vXrW0ulvo8JmFpFWqOHnBpLSbkeDLoGaY9Z8tnp/LXsJ4o3ahTnqrP8Qb2nW4ajXf6yC 6ZDg== X-Gm-Message-State: AOJu0Ywf5ihokrDZREmQ5ltVzPBOhHKmJDI2A0kezAQD9tueZZ6cCqIz OVD4dhjBuyYWJF1uylq3rwH2/Fis/g== X-Google-Smtp-Source: AGHT+IHOFdGW52whxAtpTQwrYswR+9FGN+f7HEI1ais6Hmme/G/bIwskagiWtjT5lu9yqzqehThNh6eXAg== X-Received: from elver.muc.corp.google.com ([2a00:79e0:9c:201:8dc0:5176:6fda:46a0]) (user=elver job=sendgmr) by 2002:a81:451d:0:b0:589:9d51:c8c0 with SMTP id s29-20020a81451d000000b005899d51c8c0mr41482ywa.2.1691767197794; Fri, 11 Aug 2023 08:19:57 -0700 (PDT) Date: Fri, 11 Aug 2023 17:18:38 +0200 Precedence: bulk X-Mailing-List: llvm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 X-Mailer: git-send-email 2.41.0.694.ge786442a9b-goog Message-ID: <20230811151847.1594958-1-elver@google.com> Subject: [PATCH v4 1/4] 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 , Arnd Bergmann , Greg Kroah-Hartman , Paul Moore , James Morris , "Serge E. Hallyn" , 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, linux-security-module@vger.kernel.org, llvm@lists.linux.dev, Dmitry Vyukov , Alexander Potapenko , kasan-dev@googlegroups.com, linux-toolchains@vger.kernel.org Content-Type: text/plain; charset="UTF-8" [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 Reviewed-by: Miguel Ojeda Reviewed-by: Nick Desaulniers Acked-by: Steven Rostedt (Google) Acked-by: Mark Rutland --- v4: * Guard attribute based on known-supported architectures to avoid compiler warnings about the attribute being ignored. 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..c523c6683789 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__) && (defined(CONFIG_X86_64) || defined(CONFIG_ARM64)) +# define __preserve_most notrace __attribute__((__preserve_most__)) +#else +# define __preserve_most +#endif + /* Builtins */ /* -- 2.41.0.694.ge786442a9b-goog