linux-kernel - Re: [PATCH RFC 2/4] include/linux/compiler.h: allow memory operands

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Message-ID: <20190107131350-mutt-send-email-mst@kernel.org>
Date:   Mon, 7 Jan 2019 13:16:50 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-kernel@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>, linux-arch@...r.kernel.org,
        netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        linux-sparse@...r.kernel.org
Subject: Re: [PATCH RFC 2/4] include/linux/compiler.h: allow memory operands

On Mon, Jan 07, 2019 at 05:54:27PM +0000, Will Deacon wrote:
> 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@...hat.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

Thanks, I'll drop this patch.