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From:   Marco Elver <elver@google.com>
Date:   Wed, 2 Aug 2023 18:50:39 +0200
Message-ID: <CANpmjNPVO_t058c6Wcwr9TBwxeoH7Ba0ECsf6Wapn60br8EtkQ@mail.gmail.com>
Subject: Re: [PATCH 1/3] Compiler attributes: Introduce the __preserve_most
 function attribute
To:     elver@google.com, Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>
Cc:     Guenter Roeck <linux@roeck-us.net>, Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        James Morse <james.morse@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Zenghui Yu <yuzenghui@huawei.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, Miguel Ojeda <ojeda@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Tom Rix <trix@redhat.com>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
        linux-kernel@vger.kernel.org, llvm@lists.linux.dev,
        Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        kasan-dev@googlegroups.com, linux-toolchains@vger.kernel.org,
        Mark Rutland <mark.rutland@arm.com>,
        Peter Zijlstra <peterz@infradead.org>
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On Wed, 2 Aug 2023 at 17:07, Marco Elver <elver@google.com> 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."
>
> [1] https://clang.llvm.org/docs/AttributeReference.html#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.
>
> Introduce the attribute to compiler_attributes.h.
>
> Signed-off-by: Marco Elver <elver@google.com>
> ---
>  include/linux/compiler_attributes.h | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
> index 00efa35c350f..615a63ecfcf6 100644
> --- a/include/linux/compiler_attributes.h
> +++ b/include/linux/compiler_attributes.h
> @@ -321,6 +321,17 @@
>  # define __pass_object_size(type)
>  #endif
>
> +/*
> + * Optional: not supported by gcc.
> + *
> + * clang: https://clang.llvm.org/docs/AttributeReference.html#preserve-most
> + */
> +#if __has_attribute(__preserve_most__)
> +# define __preserve_most __attribute__((__preserve_most__))
> +#else
> +# define __preserve_most
> +#endif

Mark says that there may be an issue with using this in combination
with ftrace because arm64 tracing relies on AAPCS. Probably not just
arm64, but also other architectures (x86?).

To make this safe, I'm going to move __preserve_most to
compiler_types.h and always pair it with notrace and some comments in
v2.