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

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Message-ID: <20200821230435.GA56974@rani.riverdale.lan>
Date:   Fri, 21 Aug 2020 19:04:35 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        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>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order

On Fri, Aug 21, 2020 at 02:37:48AM +0200, Thomas Gleixner wrote:
> On Thu, Aug 20 2020 at 09:06, Arvind Sankar wrote:
> > I don't think that's an issue, or at least, not one where force_order
> > helps.
> >
> > If the source for foo() is not visible to the compiler, the only reason
> > force_order prevents the reordering is because foo() might have
> > references to it, but equally foo() might have volatile asm, so the
> > reordering isn't possible anyway.
> >
> > If the source is visible, force_order won't prevent any reordering
> > except across references to force_order, but the only references are
> > from the volatile asm's which already prevent reordering.
> >
> > I think force_order can only help with buggy compilers, and for those it
> > should really have been an input-output operand -- it wouldn't currently
> > do anything to prevent cr writes from being reordered.
> 
> Fair enough. Care to provide a patch which has the collected wisdom of
> this thread in the changelog?
> 
> Thanks,
> 
>         tglx

The gcc bug I linked to earlier is only fixed in gcc-6 onwards. Is that
good enough to remove force_order? I can test gcc-4.9 and gcc-5 to check
if it would currently have any impact.

CBL guys, can you confirm that clang also will not reorder volatile asm?

Thanks.