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From:   Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Date:   Wed, 2 Aug 2023 19:08:51 +0200
Message-ID: <CANiq72k7_Ujg31UHfivv8zyog-6Vs7YehTpk2y-qRiuT-KMmAw@mail.gmail.com>
Subject: Re: [PATCH 1/3] Compiler attributes: Introduce the __preserve_most
 function attribute
To:     Marco Elver <elver@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        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, Aug 2, 2023 at 6:51=E2=80=AFPM Marco Elver <elver@google.com> wrote=
:
>
> 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.

Sounds good, thanks! The patch here was otherwise good in terms of
`compiler_attributes.h`.

I was also thinking about the implications for Rust. I guess if we
need to call them, we will go through a C helper for the moment, and
later on if we get cross-language LTO (or the LLVM IR hacks), it will
hopefully not be a performance penalty.

Cheers,
Miguel