linux-kernel - Re: [PATCH 1/2] MIPS: MSA: bugfix - disable MSA during thread switch correctly

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Message-ID: <20150522232019.GA10556@linux-mips.org>
Date:	Sat, 23 May 2015 01:20:19 +0200
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
Cc:	linux-mips@...ux-mips.org, rusty@...tcorp.com.au,
	alexinbeijing@...il.com, paul.burton@...tec.com,
	david.daney@...ium.com, alex@...x-smith.me.uk,
	linux-kernel@...r.kernel.org, james.hogan@...tec.com,
	markos.chandras@...tec.com, macro@...ux-mips.org,
	eunb.song@...sung.com, manuel.lauss@...il.com,
	andreas.herrmann@...iumnetworks.com
Subject: Re: [PATCH 1/2] MIPS: MSA: bugfix - disable MSA during thread switch
 correctly

On Fri, May 22, 2015 at 11:37:34AM -0700, Leonid Yegoshin wrote:

> On 05/22/2015 02:38 AM, Ralf Baechle wrote:
> >Just move the call to finish_arch_switch().
> 
> It might be a problem later, then a correct MSA partiton starts working. It
> should be tight to saving MSA registers in that case.
> 
> >Your rewrite also dropped the if (cpu_has_msa) condition from
> >disable_msa() probably causing havoc on lots of CPUs which will likely not
> >decode the set bits of the MFC0/MTC0 instructions thus end up accessing
> >Config0. Ralf
> 
> Right before this chunk of code there is a saving MSA registers. Does it
> causing a havoc or else?
> 
> May I ask you to look into switch_to macro to figure out how "if
> (cpu_has_msa)" check works in this case?

Ah sorry I now see that your added code is not executed for all CPUs but
only those having MSA.  So then it's safe.

Still I don't stylistically like defining the register t4 in the middle
of the code.

Below my suggested patch.  It's advantage is that for non-MSA platforms
the call to disable_msa() will be removed entirely.

Something like Paul's http://patchwork.linux-mips.org/patch/10111/ (assuming
it's correct and tested) seems like a full cleanup but it's way too
complex for 4.1 or the stable kernels.

  Ralf

Signed-off-by: Ralf Baechle <ralf@...ux-mips.org>

 arch/mips/include/asm/switch_to.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/mips/include/asm/switch_to.h b/arch/mips/include/asm/switch_to.h
index e92d6c4b..7163cd7 100644
--- a/arch/mips/include/asm/switch_to.h
+++ b/arch/mips/include/asm/switch_to.h
@@ -104,7 +104,6 @@ do {									\
 	if (test_and_clear_tsk_thread_flag(prev, TIF_USEDMSA))		\
 		__fpsave = FP_SAVE_VECTOR;				\
 	(last) = resume(prev, next, task_thread_info(next), __fpsave);	\
-	disable_msa();							\
 } while (0)
 
 #define finish_arch_switch(prev)					\
@@ -122,6 +121,7 @@ do {									\
 	if (cpu_has_userlocal)						\
 		write_c0_userlocal(current_thread_info()->tp_value);	\
 	__restore_watch();						\
+	disable_msa();							\
 } while (0)
 
 #endif /* _ASM_SWITCH_TO_H */
--
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