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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Arvind Sankar <nivedita@alum.mit.edu>,
        "Paul E. McKenney" <paulmck@kernel.org>
Cc:     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: <20200813180933.GA532283@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>
Date:   Thu, 20 Aug 2020 12:44:06 +0200
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On Thu, Aug 13 2020 at 14:09, Arvind Sankar wrote:
> On Thu, Aug 13, 2020 at 10:37:01AM -0700, Paul E. McKenney wrote:
>> > Let me ask (hopefully) useful questions this time:
>> > 
>> >   Is a compiler allowed to reorder two 'asm volatile()'?
>> > 
>> >   Are there compilers (gcc >= 4.9 or other supported ones) which do that?
>> 
>> I would hope that the answer to both of these questions is "no"!
>> 
>> But I freely confess that I have been disappointed before on this sort
>> of thing.  :-/
>> 
>> 							Thanx, Paul
>
> Ok, I found this, so gcc developers consider re-ordering volatile asm
> wrt each other a bug at least.
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602

Yes. It prevents reordering of volatiles, but it does not necessarily
prevent reorder of something like this:

        asm volatile(...);
        foo();
        asm volatile(...);

it might turn that into

        foo();
        asm volatile(...);
        asm volatile(...);

if I understood their discussion correctly. So removing this magic is
not really straight forward.

Thanks,

        tglx