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

[Kamal Mostafa]

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 mxcsr.daz

to the linux-3.8.y-queue branch of the 3.8.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-3.8.y-queue

This patch is scheduled to be released in version 3.8.13.7.

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

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 31f562ab763775a754dee7b73a6c6740fffe43df 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: x86, fpu: correct the asm constraints for fxsave, unbreak mxcsr.daz
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
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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: Kamal Mostafa <kamal 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 cb33909..f7ea30d 100644
--- a/arch/x86/kernel/i387.c
+++ b/arch/x86/kernel/i387.c
@@ -116,7 +116,7 @@ static void __cpuinit 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;
--
1.8.1.2