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From: Segher Boessenkool <segher@kernel.crashing.org>
To: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
	Sedat Dilek <sedat.dilek@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Nick Desaulniers <ndesaulniers@google.com>,
	"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>,
	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>,
	nadav.amit@gmail.com,
	Nathan Chancellor <natechancellor@gmail.com>
Subject: Re: [PATCH v2] x86/asm: Replace __force_order with memory clobber
Date: Wed, 2 Sep 2020 12:16:24 -0500
Message-ID: <20200902171624.GX28786@gate.crashing.org> (raw)
In-Reply-To: <20200902153346.3296117-1-nivedita@alum.mit.edu>

On Wed, Sep 02, 2020 at 11:33:46AM -0400, Arvind Sankar wrote:
> The CRn accessor functions use __force_order as a dummy operand to
> prevent the compiler from reordering the inline asm.
> 
> The fact that the asm is volatile should be enough to prevent this
> already, however older versions of GCC had a bug that could sometimes
> result in reordering. This was fixed in 8.1, 7.3 and 6.5. Versions prior
> to these, including 5.x and 4.9.x, may reorder volatile asm.

Reordering them amongst themselves.  Yes, that is bad.  Reordering them
with "random" code is Just Fine.

Volatile asm should be executed on the real machine exactly as often as
on the C abstract machine, and in the same order.  That is all.

> + * The compiler should not reorder volatile asm,

So, this comment needs work.  And perhaps the rest of the patch as well?


Segher

  parent reply index

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
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 [this message]
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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