From: "Paul E. McKenney" <paulmck@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
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
Date: Thu, 13 Aug 2020 10:37:01 -0700 [thread overview]
Message-ID: <20200813173701.GC4295@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <87h7t6tpye.fsf@nanos.tec.linutronix.de>
On Thu, Aug 13, 2020 at 07:28:57PM +0200, Thomas Gleixner wrote:
> 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?
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
next prev parent reply other threads:[~2020-08-13 17:37 UTC|newest]
Thread overview: 63+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-27 13:53 [PATCH] x86: work around clang IAS bug referencing __force_order Arnd Bergmann
2020-08-01 11:50 ` Sedat Dilek
2020-08-06 22:13 ` Thomas Gleixner
2020-08-07 7:03 ` Sedat Dilek
2020-08-04 0:09 ` Nick Desaulniers
2020-08-14 17:29 ` Sedat Dilek
2020-08-14 21:19 ` Sedat Dilek
2020-08-14 22:57 ` Nick Desaulniers
2020-08-15 0:26 ` Nick Desaulniers
2020-08-15 3:28 ` Sedat Dilek
2020-08-15 8:23 ` Sedat Dilek
2020-08-15 10:46 ` Sedat Dilek
2020-08-15 14:39 ` Sedat Dilek
2020-08-16 9:37 ` Sedat Dilek
2020-08-06 22:11 ` Thomas Gleixner
2020-08-13 0:12 ` Nick Desaulniers
2020-08-13 8:49 ` David Laight
2020-08-13 17:20 ` Arvind Sankar
2020-08-13 17:28 ` Thomas Gleixner
2020-08-13 17:37 ` Paul E. McKenney [this message]
2020-08-13 18:09 ` Arvind Sankar
2020-08-13 18:20 ` Paul E. McKenney
2020-08-20 10:44 ` Thomas Gleixner
2020-08-20 13:06 ` Arvind Sankar
2020-08-21 0:37 ` Thomas Gleixner
2020-08-21 23:04 ` Arvind Sankar
2020-08-21 23:16 ` Nick Desaulniers
2020-08-21 23:25 ` Arvind Sankar
2020-08-22 0:43 ` Thomas Gleixner
2020-08-22 3:55 ` Arvind Sankar
2020-08-22 8:41 ` Segher Boessenkool
2020-08-22 9:23 ` Sedat Dilek
2020-08-22 9:51 ` Sedat Dilek
2020-08-22 10:26 ` Segher Boessenkool
2020-08-22 10:35 ` Arnd Bergmann
2020-08-22 18:17 ` Miguel Ojeda
2020-08-22 21:08 ` Linus Torvalds
2020-08-22 23:10 ` Arvind Sankar
2020-08-23 0:10 ` Linus Torvalds
2020-08-23 1:16 ` Arvind Sankar
2020-08-23 21:25 ` [PATCH] x86/asm: Replace __force_order with memory clobber Arvind Sankar
2020-08-24 17:50 ` Nathan Chancellor
2020-08-24 19:13 ` Miguel Ojeda
2020-08-25 15:19 ` Arvind Sankar
2020-08-25 15:21 ` Sedat Dilek
2020-09-02 15:33 ` [PATCH v2] " Arvind Sankar
2020-09-02 15:58 ` David Laight
2020-09-02 16:14 ` Arvind Sankar
2020-09-02 16:08 ` Arvind Sankar
2020-09-02 20:26 ` David Laight
2020-09-02 17:16 ` Segher Boessenkool
2020-09-02 17:36 ` Arvind Sankar
2020-09-02 18:19 ` Miguel Ojeda
2020-09-02 18:24 ` Arvind Sankar
2020-09-02 23:21 ` [PATCH v3] " Arvind Sankar
2020-09-03 2:17 ` Kees Cook
2020-09-03 5:34 ` Miguel Ojeda
2020-09-30 20:50 ` Kees Cook
2020-10-01 10:12 ` [tip: x86/asm] x86/asm: Replace __force_order with a " tip-bot2 for Arvind Sankar
2020-10-13 9:30 ` tip-bot2 for Arvind Sankar
2020-08-22 21:17 ` [PATCH] x86: work around clang IAS bug referencing __force_order Arvind Sankar
2020-08-23 13:31 ` David Laight
2020-09-08 22:25 ` Pavel Machek
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