linux-kernel - Re: [PATCH v2] x86/asm: Replace __force

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Date:   Wed, 2 Sep 2020 12:16:24 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        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>,
        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>, nadav.amit@...il.com,
        Nathan Chancellor <natechancellor@...il.com>
Subject: Re: [PATCH v2] x86/asm: Replace __force_order with memory clobber

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