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Date:   Sat, 22 Aug 2020 19:10:55 -0400
From:   Arvind Sankar <>
To:     Linus Torvalds <>
Cc:     Miguel Ojeda <>,
        Sedat Dilek <>,
        Segher Boessenkool <>,
        Arvind Sankar <>,
        Thomas Gleixner <>,
        Nick Desaulniers <>,
        "Paul E. McKenney" <>,
        Ingo Molnar <>, Arnd Bergmann <>,
        Borislav Petkov <>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <>,
        "H. Peter Anvin" <>,
        "Kirill A. Shutemov" <>,
        Zhenzhong Duan <>,
        Kees Cook <>,
        Peter Zijlstra <>,
        Juergen Gross <>,
        Andy Lutomirski <>,
        Andrew Cooper <>,
        LKML <>,
        clang-built-linux <>,
        Will Deacon <>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order

On Sat, Aug 22, 2020 at 02:08:27PM -0700, Linus Torvalds wrote:
> 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

Actually, is a memory clobber required for correctness? Memory accesses
probably shouldn't be reordered across a CRn write. Is asm volatile
enough to stop that or do you need a memory clobber?

Replacing force_order with memory clobber introduces a few extra
instructions (testing with defconfig), but only in x86-64
hibernate/reboot/sleep code and early_ioremap_init on x86-32.

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