Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   David Laight <David.Laight@ACULAB.COM>
To:     'Segher Boessenkool' <segher@kernel.crashing.org>,
        Nicholas Piggin <npiggin@gmail.com>
CC:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Paul Mackerras" <paulus@samba.org>,
        "msuchanek@suse.de" <msuchanek@suse.de>,
        "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>
Subject: RE: [PATCH v5 20/22] powerpc/syscall: Avoid storing 'current' in
 another pointer
Thread-Topic: [PATCH v5 20/22] powerpc/syscall: Avoid storing 'current' in
 another pointer
Thread-Index: AQHW/uu6M1m95GjSlEGhmhkEL2m4wKpP4cNQ
Date:   Tue, 9 Feb 2021 14:31:16 +0000
Message-ID: <d35cc46eca474b2c9f94a4de269321e7@AcuMS.aculab.com>
References: <cover.1612796617.git.christophe.leroy@csgroup.eu>
 <24804747098369ebcdac38970b8f7a1260bdd248.1612796617.git.christophe.leroy@csgroup.eu>
 <1612838134.rvncv9kzls.astroid@bobo.none>
 <20210209135053.GD27854@gate.crashing.org>
In-Reply-To: <20210209135053.GD27854@gate.crashing.org>
Accept-Language: en-GB, en-US
MIME-Version: 1.0
Content-Language: en-US
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8BIT
Precedence: bulk

From: Segher Boessenkool
> Sent: 09 February 2021 13:51
> 
> On Tue, Feb 09, 2021 at 12:36:20PM +1000, Nicholas Piggin wrote:
> > What if you did this?
> 
> > +static inline struct task_struct *get_current(void)
> > +{
> > +	register struct task_struct *task asm ("r2");
> > +
> > +	return task;
> > +}
> 
> Local register asm variables are *only* guaranteed to live in that
> register as operands to an asm.  See
>   https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html#Local-Register-Variables
> ("The only supported use" etc.)
> 
> You can do something like
> 
> static inline struct task_struct *get_current(void)
> {
> 	register struct task_struct *task asm ("r2");
> 
> 	asm("" : "+r"(task));
> 
> 	return task;
> }
> 
> which makes sure that "task" actually is in r2 at the point of that asm.

If "r2" always contains current (and is never assigned by the compiler)
why not use a global register variable for it?

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)