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

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Message-ID: <20200813173701.GC4295@paulmck-ThinkPad-P72>
Date:   Thu, 13 Aug 2020 10:37:01 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     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 Thu, Aug 13, 2020 at 07:28:57PM +0200, Thomas Gleixner wrote:
> Nick Desaulniers <ndesaulniers@...gle.com> writes:
> > On Thu, Aug 6, 2020 at 3:11 PM Thomas Gleixner <tglx@...utronix.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?

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