Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <87h7t6tpye.fsf@nanos.tec.linutronix.de> <20200813173701.GC4295@paulmck-ThinkPad-P72>
 <20200813180933.GA532283@rani.riverdale.lan> <875z9dioll.fsf@nanos.tec.linutronix.de>
 <20200820130641.GA536306@rani.riverdale.lan> <87zh6ohm03.fsf@nanos.tec.linutronix.de>
 <20200821230435.GA56974@rani.riverdale.lan> <CAKwvOdkoB+fT9tt7vgg1R6J-NEr77EWP5X8nFat_L-HvEzWGzA@mail.gmail.com>
 <87eenzqzmr.fsf@nanos.tec.linutronix.de> <20200822035552.GA104886@rani.riverdale.lan>
 <20200822084133.GL28786@gate.crashing.org> <CA+icZUVf9a8LC2Mf0cFymYQfALWs6bVfR1v1nRbxKaLAyU6NkQ@mail.gmail.com>
 <CA+icZUWGHo3mkN4=ZYeU37p8KJhxTXS5QKcAvq_HaqTHmFkw2g@mail.gmail.com> <CANiq72=3iOh5Z_rboZf-_GUpBE2LUUv3yPKOVNN_XmaNswdNSw@mail.gmail.com>
In-Reply-To: <CANiq72=3iOh5Z_rboZf-_GUpBE2LUUv3yPKOVNN_XmaNswdNSw@mail.gmail.com>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 22 Aug 2020 14:08:27 -0700
Message-ID: <CAHk-=whaVW4FQjdwaicLFE4kiqr18rk6V50CuU-ziUPyRFjHrg@mail.gmail.com>
Subject: Re: [PATCH] x86: work around clang IAS bug referencing __force_order
To:     Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc:     Sedat Dilek <sedat.dilek@gmail.com>,
        Segher Boessenkool <segher@kernel.crashing.org>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        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>,
        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>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Sat, Aug 22, 2020 at 11:17 AM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> However, the important question is whether those users/companies care
> about running the latest kernels. Many of those definitely do not want
> to touch their kernel either. For those that do, there are several
> longterms to pick from that still support 4.9, as well as other
> workarounds.
>
> Thus I am usually in favor of raising the minimum whenever new hacks
> are required to be added. On the other hand, we already raised the
> version twice this year and it is not clear to me what is the minimum
> version we would need to go for to ensure this does not bite us.

Yeah. The good news is that I haven't seen a lot of pushback on the
gcc version updates so far. I was expecting some complaints. I haven't
seen a single one.

That may be because people did end up finding it very onerous and
complained internally on channels I haven't seen, but it might also be
indicative of us having perhaps been a bit too timid about compiler
version updates.

However, in this case, can we just leave that old "__force_order" hack
alone, and to work around the clang thing, just make a dummy
definition of it anyway.

Alternatively, just use the memory clobber. We use memory clobbers
elsewhere in inline asms to make sure they are serialized, it's not
normally a huge problem. Both clang and gcc should be smart enough to
know that a memory clobber doesn't matter for things like local
variables etc that might be on stack but have never had their address
taken.

Or are there other cases than that particular __force_order thing that
people now worry about?

             Linus