Re: [PATCH] ARM: reinsert ARCH_MULTI_V4 Kconfig option

[Uwe Kleine-König]

On Wed, Apr 09, 2014 at 04:54:16PM +0200, Jonas Jensen wrote:
> On 13 December 2013 12:39, Russell King - ARM Linux
> <linux@xxxxxxxxxxxxxxxx> 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
>  #else
>         mov     pc, \reg
> 
> 
> With this applied, and the following two possibilities in next-20140403:
> 
> [1] ARCH_MULTI_V4 only (no CONFIG_ARM_THUMB)
>      CONFIG_CPU_32v4=y
> 
> [2] ARCH_MULTI_V4 and ARCH_MULTI_V4T
>      CONFIG_ARM_THUMB=y
>      CONFIG_CPU_32v4=y
>      CONFIG_CPU_32v4T=y
> 
> 
> Booting a kernel with either option [1] or [2] yield the following results:
> 
> [1] works
> [2] hangs after "[    2.730000] Freeing unused kernel memory: 104K
> (c02f5000 - c030f000)"
> 
> 
> Any help why the moveq doesn't work would be much appreciated.
doing s/moveq/mov/ does the trick on the machine in question, but this
is obviously not an option for mainline. But it means that even on this
non-Thumb capable machine \reg contains an address that is not aligned.

Where does \reg come from? Is it provided by userspace? If so, is it a
userspace bug?

Best regards
Uwe

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