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From:   Sedat Dilek <sedat.dilek@gmail.com>
Date:   Tue, 25 Aug 2020 17:21:52 +0200
Message-ID: <CA+icZUUrsG-Y_3Rg3a6+5mXXoh_w=QYnCFAazKane9fwXAWynw@mail.gmail.com>
Subject: Re: [PATCH] x86/asm: Replace __force_order with memory clobber
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        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>,
        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>
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On Tue, Aug 25, 2020 at 5:19 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> 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.

Thanks for taking care and your patch.

I have tested this on top of Linux v5.9-rc2 with LLVM toolchain
v11.0.0-rc2 (ThinLTO).

Tested-by: Sedat Dilek <sedat.dilek@gmail.com>

- Sedat -