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

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Message-ID: <20140409150640.GQ16119@n2100.arm.linux.org.uk>
Date:	Wed, 9 Apr 2014 16:06:40 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Jonas Jensen <jonas.jensen@...il.com>
Cc:	"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>, u.kleine-koenig@...gutronix.de
Subject: Re: [PATCH] ARM: reinsert ARCH_MULTI_V4 Kconfig option

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?

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