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

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Date:   Sat, 22 Aug 2020 19:10:55 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        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 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.