commit 7f72cb082501852ae9ebe403efef2ebb0437ff57
author H.J. Lu <hjl.tools@gmail.com> Fri Jul 26 09:11:56 2013 -0700
committer Greg Kroah-Hartman <gregkh@linuxfoundation.org> Sun Aug 11 15:38:43 2013 -0700
tree e37574e9a6a93eb7fd95c93dc58bec446d2f03ce
parent 5cbff87c00cf1b8e3a077465e4ec501dd21651c8

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

commit eaa5a990191d204ba0f9d35dbe5505ec2cdd1460 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/kernel/i387.c

diff --git a/arch/x86/kernel/i387.c b/arch/x86/kernel/i387.c
index 2d6e649..6610e81 100644
--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -132,7 +132,7 @@
 	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;