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From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Sat, 22 Aug 2020 19:10:55 -0400
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Segher Boessenkool <segher@kernel.crashing.org>,
        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>, Arnd Bergmann <arnd@arndb.de>,
        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>,
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        Will Deacon <will@kernel.org>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order
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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.