linux-kernel - Re: [PATCH] ARM: reinsert ARCH_MULTI

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Date:	Wed, 9 Apr 2014 17:13:26 +0200
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Jonas Jensen <jonas.jensen@...il.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"arm@...nel.org" <arm@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	ulli.kroll@...glemail.com, Arnd Bergmann <arnd@...db.de>,
	Olof Johansson <olof@...om.net>
Subject: Re: [PATCH] ARM: reinsert ARCH_MULTI_V4 Kconfig option

Hello Russell,

On Wed, Apr 09, 2014 at 04:06:40PM +0100, Russell King - ARM Linux wrote:
> On Wed, Apr 09, 2014 at 04:54:16PM +0200, Jonas Jensen wrote:
> > On 13 December 2013 12:39, Russell King - ARM Linux
> > <linux@....linux.org.uk> wrote:
> > > I see what's causing this: the kuser helpers are using "bx lr" to return
> > > which will be undefined on non-Thumb CPUs.  We generally cope fine with
> > > non-Thumb CPUs, conditionalising where necessary on HWCAP_THUMB or the
> > > T bit in the PSR being set.
> > >
> > > However, it looks like the kuser helpers got missed.  As a check, please
> > > look at arch/arm/kernel/entry-armv.S, find the line with:
> > >
> > >         .macro  usr_ret, reg
> > >
> > > and ensure that the mov pc, \reg case always gets used.  Please report
> > > back.
> > 
> > Uwe and Arnd came up with a solution except it doesn't work when I test it.
> > 
> > The suggested patch is:
> > 
> > diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> > index 1879e8d..de15bfd 100644
> > --- a/arch/arm/kernel/entry-armv.S
> > +++ b/arch/arm/kernel/entry-armv.S
> > @@ -739,6 +739,18 @@ ENDPROC(__switch_to)
> > 
> >         .macro  usr_ret, reg
> >  #ifdef CONFIG_ARM_THUMB
> > +       /*
> > +        * Having CONFIG_ARM_THUMB isn't a guarantee that the cpu has support
> > +        * for Thumb and so the bx instruction. Use a mov if the address to
> > +        * jump to is 32 bit aligned. (Note that this code is compiled in ARM
> > +        * mode, so this is the right test.)
> > +        */
> > +#if defined(CONFIG_CPU_32v4)
> > +       tst     \reg, #3
> > +       moveq   pc, \reg
> > +       b       .
> > +#endif
> > +
> >         bx      \reg
> 
> What's wrong with:
> 	tst	\reg, #3
> 	moveq	pc, \reg
> 	bx	\reg
> 
> rather than ending in an infinite loop?
The added b . was a test to check if the machine then hangs instead of
crashing. (And yes, that was the case, so it was tried to return to a
non-aligned address.)

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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