Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 7 Jan 2019 17:54:27 +0000
From:   Will Deacon <will.deacon@arm.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     linux-kernel@vger.kernel.org, Jason Wang <jasowang@redhat.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        Andrea Parri <andrea.parri@amarulasolutions.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        David Howells <dhowells@redhat.com>,
        Jade Alglave <j.alglave@ucl.ac.uk>,
        Luc Maranget <luc.maranget@inria.fr>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Akira Yokosawa <akiyks@gmail.com>,
        Daniel Lustig <dlustig@nvidia.com>, linux-arch@vger.kernel.org,
        netdev@vger.kernel.org, virtualization@lists.linux-foundation.org,
        Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
        linux-sparse@vger.kernel.org
Subject: Re: [PATCH RFC 2/4] include/linux/compiler.h: allow memory operands
Message-ID: <20190107175427.GB22911@fuggles.cambridge.arm.com>
References: <20190102205715.14054-1-mst@redhat.com>
 <20190102205715.14054-3-mst@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190102205715.14054-3-mst@redhat.com>
User-Agent: Mutt/1.11.1+86 (6f28e57d73f2) ()
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Jan 02, 2019 at 03:57:54PM -0500, Michael S. Tsirkin wrote:
> We don't really care whether the variable is in-register
> or in-memory. Relax the constraint accordingly.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  include/linux/compiler.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 1ad367b4cd8d..6601d39e8c48 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -154,7 +154,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>  #ifndef OPTIMIZER_HIDE_VAR
>  /* Make the optimizer believe the variable can be manipulated arbitrarily. */
>  #define OPTIMIZER_HIDE_VAR(var)						\
> -	__asm__ ("" : "=r" (var) : "0" (var))
> +	__asm__ ("" : "=rm" (var) : "0" (var))
>  #endif

I think this can break for architectures with write-back addressing modes
such as arm, where the "m" constraint is assumed to be evaluated precisely
once in the asm block.

Will