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

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Message-ID: <b87e210138ac48018613529db3b54462@AcuMS.aculab.com>
Date:   Sun, 23 Aug 2020 13:31:03 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Arvind Sankar' <nivedita@...m.mit.edu>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
CC:     Sedat Dilek <sedat.dilek@...il.com>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        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>
Subject: RE: [PATCH] x86: work around clang IAS bug referencing __force_order

From: Arvind Sankar
> Sent: 22 August 2020 22:17
...
> Assuming we don't want to risk removing force_order, I'd suggest
> - make it an input/output operand, so it enforces ordering fully.
> - either restrict it to gcc < 8, or just provide a proper definition in
>   some file (maybe arch/x86/kernel/cpu/common.c)?

Is it possible to replace __force_order with a symbol that the
linker scripts defines?
Or just define __force_order in the linker script to some
'random' constant (eg 0).

ISTM that adding "m"(__force_order) to asm volatile can do no harm.
Especially for accesses to CRn and MSRn (etc) which might have obscure
side effects.

	David

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