From: Arvind Sankar <nivedita@alum.mit.edu>
To: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc: Arvind Sankar <nivedita@alum.mit.edu>,
Linus Torvalds <torvalds@linux-foundation.org>,
Sedat Dilek <sedat.dilek@gmail.com>,
Segher Boessenkool <segher@kernel.crashing.org>,
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 14:24:49 -0400 [thread overview]
Message-ID: <20200902182449.GA3586123@rani.riverdale.lan> (raw)
In-Reply-To: <CANiq72=jwj0xocJ7iAkntbnHDQphL79XcPREz1qpW0yqrMsz4g@mail.gmail.com>
On Wed, Sep 02, 2020 at 08:19:25PM +0200, Miguel Ojeda wrote:
> On Wed, Sep 2, 2020 at 5:33 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
> >
> > + * The compiler should not reorder volatile asm, however older versions of GCC
> > + * had a bug (which was fixed in 8.1, 7.3 and 6.5) where they could sometimes
> > + * reorder volatile asm. The write functions are not a problem since they have
> > + * memory clobbers preventing reordering. To prevent reads from being reordered
> > + * with respect to writes, use a dummy memory operand.
>
> I see you added the information to the commit message, but I'd still
> reword this to something like:
Ah sorry, I forgot to change the comment in the source as well.
>
> "The compiler should not reorder volatile asm, however GCC 4.9.x and
> 5.x have a bug where they could sometimes reorder volatile asm. The
> bug was fixed in 8.1, 7.3 and 6.5. The write functions are not a
> problem since they have memory clobbers preventing reordering. To
> prevent reads from being reordered with respect to writes, use a dummy
> memory operand."
>
> The important point is that 4.9.x and 5.x *have* the bug and that is
> the reason for having the hack. In the old wording it seems like the
> bug is no more. Then one wonders why the hack is still there (i.e.
> perhaps because we don't trust it, perhaps to support the rest of the
> minor versions which are newer, perhaps to avoid regressions, perhaps
> only the comment was updated, etc.).
>
> Cheers,
> Miguel
next prev parent reply other threads:[~2020-09-02 18:24 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
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 [this message]
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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