linux-kernel - [PATCH 48/75] x86, fpu: correct the asm constraints for fxsave, unbreak mxcsr.daz

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Message-Id: <1376476624-10011-49-git-send-email-luis.henriques@canonical.com>
Date:	Wed, 14 Aug 2013 11:36:37 +0100
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 48/75] x86, fpu: correct the asm constraints for fxsave, unbreak mxcsr.daz

3.5.7.19 -stable 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: Luis Henriques <luis.henriques@...onical.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

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