[ 3.5.y.z extended stable ] Patch "x86, fpu: correct the asm constraints for fxsave, unbreak" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    x86, fpu: correct the asm constraints for fxsave, unbreak

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From d0dd9ab717ceddeef22a1cd038676048388f50ec Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools at gmail.com>
Date: Fri, 26 Jul 2013 09:11:56 -0700
Subject: [PATCH] x86, fpu: correct the asm constraints for fxsave, unbreak
 mxcsr.daz
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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 at linux.intel.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 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;
--
1.8.3.2