x86, fpu: correct the asm constraints for fxsave, unbreak mxcsr.daz

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 <hpa@linux.intel.com>
Cc: <stable@vger.kernel.org>

diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
index 202d24f..5d576ab 100644 (file)
--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -116,7 +116,7 @@ static void mxcsr_feature_mask_init(void)
 
        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;