From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 44345C001DF for ; Tue, 8 Aug 2023 12:36:14 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=hcvrS4CXkrsgEX1kBQK+Sj/DwjSnmxs1ktA6puAUcZA=; b=YrQ/cr61aHP0ap +BrMyUGjrHiOU+miDEejD+zKVeJqUP8xhyOwnvZZe6H57pZEru7bitFwj+EjM6kxgbngFvmxkAjfS 0ZsD7JR7HBQ9wnzgG45ynaTM626sWiqeoEYX8XNejr6Unk9pUli2g9ZFy3yMlqNKrBuUIwXdHqgVx ZdNNoeVjFDyQOzxN1o1WWc16fAo8MEWyhm5NwsrdVD7UqFpjvrYYcVZhrY7MLsjk/6A8k8l3NtiI0 nH1lPKbMPdBV0uU2lPM8sgETMtCbCHWgpvB1X7cPbPDeGk/5G5ACoXPwueU9n+mOLb/FRckP4N8RX xY9NstQ0VwekIfEy1M1w==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.96 #2 (Red Hat Linux)) id 1qTLw9-002ZOQ-0Z; Tue, 08 Aug 2023 12:35:45 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.96 #2 (Red Hat Linux)) id 1qTLw5-002ZMv-29 for linux-arm-kernel@lists.infradead.org; Tue, 08 Aug 2023 12:35:43 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 681661576; Tue, 8 Aug 2023 05:36:21 -0700 (PDT) Received: from FVFF77S0Q05N.cambridge.arm.com (FVFF77S0Q05N.cambridge.arm.com [10.1.35.148]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 36E633F59C; Tue, 8 Aug 2023 05:35:35 -0700 (PDT) Date: Tue, 8 Aug 2023 13:35:29 +0100 From: Mark Rutland To: Marco Elver Cc: Andrew Morton , Kees Cook , Guenter Roeck , Peter Zijlstra , 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 Subject: Re: [PATCH v3 1/3] compiler_types: Introduce the Clang __preserve_most function attribute Message-ID: References: <20230808102049.465864-1-elver@google.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20230808102049.465864-1-elver@google.com> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20230808_053541_801775_4A6BADA8 X-CRM114-Status: GOOD ( 35.09 ) 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: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Aug 08, 2023 at 12:17:25PM +0200, Marco Elver wrote: > [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) So long as this implies notrace I believe it is safe, so: Acked-by: Mark Rutland Mark. > --- > 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 */ > > /* > -- > 2.41.0.640.ga95def55d0-goog > _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel