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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Nick Desaulniers <ndesaulniers@google.com>,
        Ingo Molnar <mingo@redhat.com>
Cc:     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>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Will Deacon <will@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order
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Date:   Thu, 13 Aug 2020 19:28:57 +0200
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Nick Desaulniers <ndesaulniers@google.com> writes:
> On Thu, Aug 6, 2020 at 3:11 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>> > + *
>> > + * Clang sometimes fails to kill the reference to the dummy variable, so
>> > + * provide an actual copy.
>>
>> Can that compiler be fixed instead?
>
> I don't think so. The logic in the compiler whether to emit an

Forget that I asked. Heat induced brain damaged.

> I'd much rather remove all of __force_order.

Right.

> Not sure about the comment in arch/x86/include/asm/special_insns.h
> either; smells fishy like a bug with a compiler from a long time ago.
> It looks like it was introduced in:
> commit d3ca901f94b32 ("x86: unify paravirt parts of system.h")
> Lore has this thread:
> https://lore.kernel.org/lkml/4755A809.4050305@qumranet.com/
> Patch 4: https://lore.kernel.org/lkml/11967844071346-git-send-email-gcosta@redhat.com/
> It seems like there was a discussion about %cr8, but no one asked
> "what's going on here with __force_order, is that right?"

Correct and the changelog is uselss in this regard.

> Quick boot test of the below works for me, though I should probably
> test hosting a virtualized guest since d3ca901f94b32 refers to
> paravirt.  Thoughts?

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?

Thanks,

        tglx