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From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Tue, 25 Aug 2020 11:19:10 -0400
To:     Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Cc:     Arvind Sankar <nivedita@alum.mit.edu>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Segher Boessenkool <segher@kernel.crashing.org>,
        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>,
        Kees Cook <keescook@chromium.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Juergen Gross <jgross@suse.com>,
        Andy Lutomirski <luto@kernel.org>,
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        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Will Deacon <will@kernel.org>
Subject: Re: [PATCH] x86/asm: Replace __force_order with memory clobber
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On Mon, Aug 24, 2020 at 09:13:34PM +0200, Miguel Ojeda wrote:
> Hi Arvind,
> 
> On Sun, Aug 23, 2020 at 11:25 PM Arvind Sankar <nivedita@alum.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@alum.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.
> 

Ok.

> > + * 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.
> 

Ok.

> > + * 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?
> 

This was intentional, but I can remove it if people don't like the extra
whitespace.

I'll wait a few days for additional review comments before sending v2.

Thanks.