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

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Date:   Mon, 24 Aug 2020 21:13:34 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        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>
Subject: Re: [PATCH] x86/asm: Replace __force_order with memory clobber

Hi Arvind,

On Sun, Aug 23, 2020 at 11:25 PM Arvind Sankar <nivedita@...m.mit.edu> wrote:
>
> - Using a dummy input operand with an arbitrary constant address for the
>   read functions, instead of a global variable. This will prevent reads
>   from being reordered across writes, while allowing memory loads to be
>   cached/reordered across CRn reads, which should be safe.

Assuming no surprises from compilers, this looks better than dealing
with different code for each compiler.

> Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602

A lore link to the other discussion would be nice here for context.

> + * 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

I'd mention the state of GCC 5 here.

> + * 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.
>   */
> -extern unsigned long __force_order;
> +

Spurious newline?

Cheers,
Miguel