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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Sat, 22 Aug 2020 12:35:14 +0200
Message-ID: <CAK8P3a031YiKxYYX5uaVf=y_MJZo-eNwOsbxsVnH0QdoZznNFA@mail.gmail.com>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order
To:     Segher Boessenkool <segher@kernel.crashing.org>
Cc:     Sedat Dilek <sedat.dilek@gmail.com>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Thomas Gleixner <tglx@linutronix.de>,
        Nick Desaulniers <ndesaulniers@google.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Zhenzhong Duan <zhenzhong.duan@oracle.com>,
        Kees Cook <keescook@chromium.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Juergen Gross <jgross@suse.com>,
        Andy Lutomirski <luto@kernel.org>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Will Deacon <will@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
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On Sat, Aug 22, 2020 at 12:26 PM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
> [ There is GCC 4.9.4, no one should use an older 4.9. ]
>
> I mentioned 5 for a reason: the whole function this patch is to did not
> exist before then!  That does not mean the bug existed or did not exist
> before GCC 5, but it does for example mean that a backport to 4.9 or
> older isn't trivial at all.
>
> > I am asking myself who is using such ancient compilers?
>
> Some distros have a GCC 4.8 as system compiler.  We allow building GCC
> itself with a compiler that far back, for various reasons as well (and
> this is very sharp already, the last mainline GCC 4.8 release is from
> June 2015, not all that long ago at all).
>
> But, one reason this works is because people actually test it.  Does
> anyone actually test the kernel with old compilers?  It isn't hard to
> build a new compiler (because we make sure building a newer compiler
> works with older compilers, etc. :-) ), and as you say, most distros
> have newer compilers available nowadays.

We only recently changed the minimum from 4.6 to 4.8, and
subsequently to 4.9. Most people have fairly recent compilers,
but there are a number of notable kernel developers that
intentionally stick to old versions because of compile speed.

Each major compiler release adds about 4% overhead in total time
to compile a kernel, so between gcc-4.6 and gcc-11 you add over
50% in build time.

      Arnd