linux-kernel - [037/121] x86, fpu: correct the asm constraints for fxsave, unbreak mxcsr.daz

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Message-ID: <lsq.1378608721.667859478@decadent.org.uk>
Date:	Sun, 08 Sep 2013 03:52:01 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "H.J. Lu" <hjl.tools@...il.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>
Subject: [037/121] x86, fpu: correct the asm constraints for fxsave,
 unbreak mxcsr.daz

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

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

From: "H.J. Lu" <hjl.tools@...il.com>

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@...ux.intel.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/x86/kernel/i387.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -51,7 +51,7 @@ void __cpuinit mxcsr_feature_mask_init(v
 	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;

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