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

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Date:	Wed, 9 Apr 2014 16:54:16 +0200
From:	Jonas Jensen <jonas.jensen@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
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 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
 #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.


The commit where this was tested:
https://bitbucket.org/Kasreyn/linux-next/commits/48320cbe0d5a1fb6ada52cea9efd87c7712367cb


Regards,
Jonas
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