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From: Arvind Sankar <nivedita@alum.mit.edu>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Arvind Sankar <nivedita@alum.mit.edu>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	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: Fri, 21 Aug 2020 19:25:05 -0400
Message-ID: <20200821232505.GA66405@rani.riverdale.lan> (raw)
In-Reply-To: <CAKwvOdkoB+fT9tt7vgg1R6J-NEr77EWP5X8nFat_L-HvEzWGzA@mail.gmail.com>

On Fri, Aug 21, 2020 at 04:16:56PM -0700, Nick Desaulniers wrote:
> On Fri, Aug 21, 2020 at 4:04 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
> >
> > On Fri, Aug 21, 2020 at 02:37:48AM +0200, Thomas Gleixner wrote:
> > > 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.
> 
> I agree 100%.  From the link to GCC docs, the code in question doesn't
> even follow the pattern from the doc from informing the compiler of
> any dependency, it just looks like !@#$.
> 
> > >
> > > Fair enough. Care to provide a patch which has the collected wisdom of
> > > this thread in the changelog?
> > >
> > > Thanks,
> > >
> > >         tglx
> >
> > The gcc bug I linked to earlier is only fixed in gcc-6 onwards. Is that
> 
> (based on https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602#c14)

I actually checked gcc's git repo too. The fix is not there in gcc-4.9
and gcc-5.

> 
> > good enough to remove force_order? I can test gcc-4.9 and gcc-5 to check
> > if it would currently have any impact.
> 
> I think checking the disassemblies with a pre-gcc-6 would be good
> enough then; that bug isn't specific to this particular case.
> 
> > CBL guys, can you confirm that clang also will not reorder volatile asm?
> 
> Full disassemblies of vmlinux pre vs post __force_order removal are
> the same.  That's pretty good actually; I was worried for a code base
> of this size whether two compiles would produce the exact same
> disassemblies; I know the version strings are timestamped, for
> instance, but I didn't compare data, just .text.  I should triple
> check i386, and some of the ko's that use modified functions.  I'd be
> happy to help provide a tested by tag for numerous configurations with
> Clang.
> 
> Attaching the diff I was testing, feel free to add a commit message.
> --
> Thanks,
> ~Nick Desaulniers

Thanks, will write it up over the weekend.

  reply index

Thread overview: 63+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-27 13:53 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
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 [this message]
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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