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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Arvind Sankar <nivedita@alum.mit.edu>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        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>,
        Kees Cook <keescook@chromium.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Juergen Gross <jgross@suse.com>,
        Andy Lutomirski <luto@kernel.org>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Will Deacon <will@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order
In-Reply-To: <20200820130641.GA536306@rani.riverdale.lan>
References: <20200527135329.1172644-1-arnd@arndb.de> <878serh1b9.fsf@nanos.tec.linutronix.de> <CAKwvOdnOh3H3ga2qpTktywvcgfXW5QJaB7r4XMhigmDzLhDNeA@mail.gmail.com> <87h7t6tpye.fsf@nanos.tec.linutronix.de> <20200813173701.GC4295@paulmck-ThinkPad-P72> <20200813180933.GA532283@rani.riverdale.lan> <875z9dioll.fsf@nanos.tec.linutronix.de> <20200820130641.GA536306@rani.riverdale.lan>
Date:   Fri, 21 Aug 2020 02:37:48 +0200
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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