[154/251] x86, fpu: correct the asm constraints for fxsave, unbreak mxcsr.daz

[Steven Rostedt]

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

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

From: "H.J. Lu" <hjl.to...@gmail.com>

[ Upstream commit eaa5a990191d204ba0f9d35dbe5505ec2cdd1460 ]

GCC will optimize mxcsr_feature_mask_init in arch/x86/kernel/i387.c:

                memset(&fx_scratch, 0, sizeof(struct i387_fxsave_struct));
                asm volatile("fxsave %0" : : "m" (fx_scratch));
                mask = fx_scratch.mxcsr_mask;
                if (mask == 0)
                        mask = 0x0000ffbf;

to

                memset(&fx_scratch, 0, sizeof(struct i387_fxsave_struct));
                asm volatile("fxsave %0" : : "m" (fx_scratch));
                mask = 0x0000ffbf;

since asm statement doesn’t say it will update fx_scratch.  As the
result, the DAZ bit will be cleared.  This patch fixes it. This bug
dates back to at least kernel 2.6.12.

Signed-off-by: H. Peter Anvin <h...@linux.intel.com>
Cc: <sta...@vger.kernel.org>
Signed-off-by: Steven Rostedt <rost...@goodmis.org>
---
 arch/x86/kernel/i387.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
index f250431..6c5a7dc 100644
--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -132,7 +132,7 @@ static void __cpuinit mxcsr_feature_mask_init(void)
        clts();
        if (cpu_has_fxsr) {
                memset(&fx_scratch, 0, sizeof(struct i387_fxsave_struct));
-               asm volatile("fxsave %0" : : "m" (fx_scratch));
+               asm volatile("fxsave %0" : "+m" (fx_scratch));
                mask = fx_scratch.mxcsr_mask;
                if (mask == 0)
                        mask = 0x0000ffbf;
-- 
1.7.10.4


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