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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 8 Dec 2023 09:08:47 -0800
Message-ID: <CAKwvOdkgDwnC_jaGjXjk9yKYo=zWDR_3x7Drw3i=KX0Wyij6ew@mail.gmail.com>
Subject: Re: [PATCH 5/7] rust: file: add `Kuid` wrapper
To:     Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc:     comex <comexk@gmail.com>, Christian Brauner <brauner@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Alice Ryhl <aliceryhl@google.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Benno Lossin <benno.lossin@proton.me>,
        Andreas Hindborg <a.hindborg@samsung.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= <arve@android.com>,
        Todd Kjos <tkjos@android.com>,
        Martijn Coenen <maco@android.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Carlos Llamas <cmllamas@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Kees Cook <keescook@chromium.org>,
        Matthew Wilcox <willy@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Daniel Xu <dxu@dxuuu.xyz>, linux-kernel@vger.kernel.org,
        rust-for-linux@vger.kernel.org, linux-fsdevel@vger.kernel.org
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On Fri, Dec 8, 2023 at 8:19=E2=80=AFAM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> On Fri, Dec 8, 2023 at 6:28=E2=80=AFAM comex <comexk@gmail.com> wrote:
> >
> > Regarding the issue of wrappers not being inlined, it's possible to get=
 LLVM to optimize C and Rust code together into an object file, with the he=
lp of a compatible Clang and LLD:
> >
> > @ rustc -O --emit llvm-bc a.rs
> > @ clang --target=3Dx86_64-unknown-linux-gnu -O2 -c -emit-llvm -o b.bc b=
.c
> > @ ld.lld -r -o c.o a.bc b.bc
> >
> > Basically LTO but within the scope of a single object file.  This would=
 be redundant in cases where kernel-wide LTO is enabled.
> >
> > Using this approach might slow down compilation a bit due to needing to=
 pass the LLVM bitcode between multiple commands, but probably not very muc=
h.
> >
> > Just chiming in as someone not involved in Rust for Linux but familiar =
with these tools.  Perhaps this has been considered before and rejected for=
 some reason; I wouldn=E2=80=99t know.
>
> Thanks comex for chiming in, much appreciated.
>
> Yeah, this is what we have been calling the "local-LTO hack" and it
> was one of the possibilities we were considering for non-LTO kernel
> builds for performance reasons originally. I don't recall who
> originally suggested it in one of our meetings (Gary or Bj=C3=B6rn
> perhaps).
>
> If LLVM folks think LLVM-wise nothing will break, then we are happy to

On paper, nothing comes to mind.  No promises though.

From a build system perspective, I'd rather just point users towards
LTO if they have this concern.  We support full and thin lto.  This
proposal would add a third variant for just rust drivers.  Each
variation on LTO has a maintenance cost and each have had their own
distinct fun bugs in the past.  Not sure an additional variant is
worth the maintenance cost, even if it's technically feasible.

> go ahead with that (since it also solves the performance side), but it
> would be nice to know if it will always be OK to build like that, i.e.
> I think Andreas actually tried it and it seemed to work and boot, but
> the worry is whether there is something subtle that could have bad
> codegen in the future.
>
> (We will also need to worry about GCC.)
>
> Cheers,
> Miguel


--=20
Thanks,
~Nick Desaulniers