linux-kernel - Re: [PATCH] x86: work around clang IAS bug referencing __force

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Date:   Sat, 22 Aug 2020 14:08:27 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc:     Sedat Dilek <sedat.dilek@...il.com>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Zhenzhong Duan <zhenzhong.duan@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juergen Gross <jgross@...e.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order

On Sat, Aug 22, 2020 at 11:17 AM Miguel Ojeda
<miguel.ojeda.sandonis@...il.com> wrote:
>
> However, the important question is whether those users/companies care
> about running the latest kernels. Many of those definitely do not want
> to touch their kernel either. For those that do, there are several
> longterms to pick from that still support 4.9, as well as other
> workarounds.
>
> Thus I am usually in favor of raising the minimum whenever new hacks
> are required to be added. On the other hand, we already raised the
> version twice this year and it is not clear to me what is the minimum
> version we would need to go for to ensure this does not bite us.

Yeah. The good news is that I haven't seen a lot of pushback on the
gcc version updates so far. I was expecting some complaints. I haven't
seen a single one.

That may be because people did end up finding it very onerous and
complained internally on channels I haven't seen, but it might also be
indicative of us having perhaps been a bit too timid about compiler
version updates.

However, in this case, can we just leave that old "__force_order" hack
alone, and to work around the clang thing, just make a dummy
definition of it anyway.

Alternatively, just use the memory clobber. We use memory clobbers
elsewhere in inline asms to make sure they are serialized, it's not
normally a huge problem. Both clang and gcc should be smart enough to
know that a memory clobber doesn't matter for things like local
variables etc that might be on stack but have never had their address
taken.

Or are there other cases than that particular __force_order thing that
people now worry about?

             Linus