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

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Message-ID: <20200908222524.GE1005@bug>
Date:   Wed, 9 Sep 2020 00:25:24 +0200
From:   Pavel Machek <pavel@....cz>
To:     Sedat Dilek <sedat.dilek@...il.com>
Cc:     Segher Boessenkool <segher@...nel.crashing.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        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>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order

On Sat 2020-08-22 11:51:56, Sedat Dilek wrote:
> On Sat, Aug 22, 2020 at 11:23 AM Sedat Dilek <sedat.dilek@...il.com> wrote:
> >
> > On Sat, Aug 22, 2020 at 10:42 AM Segher Boessenkool
> > <segher@...nel.crashing.org> wrote:
> > >
> > > Hi Arvind,
> > >
> > > On Fri, Aug 21, 2020 at 11:55:52PM -0400, Arvind Sankar wrote:
> > > > Cc Segher.
> > > >
> > > > Segher, we were looking at gcc PR82602, where IRA could reorder volatile
> > > > asm's (reported on ARM). The fix was backported to gcc-6.
> > >
> > > I know ;-)
> > >
> > > > Do you know if
> > > > there is any reason the problem couldn't occur on x86 on older gcc
> > > > without the fix?
> > >
> > > No, I see no particular reason, at least GCC 5 seems vulnerable.  (The
> > > GCC 5 release branch was closed at the time this bug report was made,
> > > already).  There is no reason I see why it would work on x86 but fail
> > > elsewhere, either.
> > >
> >
> > [1] says:
> >
> > Current Minimal Requirements
> > ...
> > ====================== ===============  ========================================
> >         Program        Minimal version       Command to check the version
> > ====================== ===============  ========================================
> > GNU C                  4.9              gcc --version
> >
> > - Sedat -
> >
> > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/changes.rst#n32
> 
> [ CC Miguel Ojeda (Compiler Attributes maintainer) ]
> 
> There exist gcc-4.8 and gcc-4.9 for Debian/jessie where EOL was June
> 30, 2020 (see [1] and [2]).
> 
> In the latest available version "4.9.2-10+deb8u1" I see no PR82602 was
> backported (see [3] and [4]).
> 
> I am asking myself who is using such ancient compilers?
> Recently, I threw away GCC-8 from my Debian system.

I do have 4.9.2 on some systems. They work well, and are likely to compile
significantly faster than newer ones.

Please don't break them.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html