linux-kernel - Re: [PATCH] compiler: enable CONFIG_OPTIMIZE

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Date:   Tue, 1 Oct 2019 11:00:11 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Will Deacon <will@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Stefan Wahren <wahrenst@....net>,
        Kees Cook <keescook@...gle.com>
Subject: Re: [PATCH] compiler: enable CONFIG_OPTIMIZE_INLINING forcibly

On Tue, Oct 1, 2019 at 10:55 AM Russell King - ARM Linux admin
<linux@...linux.org.uk> wrote:
>
> On Tue, Oct 01, 2019 at 10:44:43AM -0700, Nick Desaulniers wrote:
> > I apologize; I don't mean to be difficult.  I would just like to avoid
> > surprises when code written with the assumption that it will be
> > inlined is not.  It sounds like we found one issue in arm32 and one in
> > arm64 related to outlining.  If we fix those two cases, I think we're
> > close to proceeding with Masahiro's cleanup, which I view as a good
> > thing for the health of the Linux kernel codebase.
>
> Except, using the C preprocessor for this turns the arm32 code into
> yuck:
>
> 1. We'd need to turn get_domain() and set_domain() into multi-line
>    preprocessor macro definitions, using the GCC ({ }) extension
>    so that get_domain() can return a value.
>
> 2. uaccess_save_and_enable() and uaccess_restore() also need to
>    become preprocessor macro definitions too.
>
> So, we end up with multiple levels of nested preprocessor macros.
> When something goes wrong, the compiler warning/error message is
> going to be utterly _horrid_.

That's why I preferred V1 of Masahiro's patch, that fixed the inline
asm not to make use of caller saved registers before calling a
function that might not be inlined.
-- 
Thanks,
~Nick Desaulniers