linux-kernel - [PATCH 3.2 41/74] powerpc/asm: Mark cr0 as clobbered in mftb()

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Message-ID: <lsq.1507552234.766931019@decadent.org.uk>
Date:   Mon, 09 Oct 2017 13:30:34 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Michael Ellerman" <mpe@...erman.id.au>,
        "Oliver O'Halloran" <oohall@...il.com>
Subject: [PATCH 3.2 41/74] powerpc/asm: Mark cr0 as clobbered in mftb()

3.2.94-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Oliver O'Halloran <oohall@...il.com>

commit 2400fd822f467cb4c886c879d8ad99feac9cf319 upstream.

The workaround for the CELL timebase bug does not correctly mark cr0 as
being clobbered. This means GCC doesn't know that the asm block changes cr0 and
might leave the result of an unrelated comparison in cr0 across the block, which
we then trash, leading to basically random behaviour.

Fixes: 859deea949c3 ("[POWERPC] Cell timebase bug workaround")
Signed-off-by: Oliver O'Halloran <oohall@...il.com>
[mpe: Tweak change log and flag for stable]
Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/powerpc/include/asm/reg.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/powerpc/include/asm/reg.h
+++ b/arch/powerpc/include/asm/reg.h
@@ -1054,7 +1054,9 @@
 				"	.llong 0\n"			\
 				"	.llong 0\n"			\
 				".previous"				\
-			: "=r" (rval) : "i" (CPU_FTR_CELL_TB_BUG)); rval;})
+			: "=r" (rval) \
+			: "i" (CPU_FTR_CELL_TB_BUG) : "cr0"); \
+			rval;})
 #else
 #define mftb()		({unsigned long rval;	\
 			asm volatile("mftb %0" : "=r" (rval)); rval;})