linux-kernel - [patch] x86, cpuid: add "volatile" to asm in native

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Message-Id: <1261009542.2745.55.camel@sbs-t61.sc.intel.com>
Date:	Wed, 16 Dec 2009 16:25:42 -0800
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>, stable@...nel.org
Subject: [patch] x86, cpuid: add "volatile" to asm in native_cpuid()

xsave_cntxt_init() does something like:

	cpuid(0xd, ..);	// find out what features FP/SSE/.. etc are supported

	xsetbv();	// enable the features known to OS

	cpuid(0xd, ..);	// find out the size of the context for features enabled

Depending on what features get enabled in xsetbv(), value of the
cpuid.eax=0xd.ecx=0.ebx changes correspondingly (representing the
size of the context that is enabled).

As we don't have volatile keyword for native_cpuid(), gcc 4.1.2
optimizes away the second cpuid and the kernel continues to use
the cpuid information obtained before xsetbv(), ultimately leading to kernel
crash on processors supporting more state than the legacy FP/SSE.

Add "volatile" for native_cpuid().

Signed-off-by: Suresh Siddha <suresh.b.siddha@...el.com>
Cc: stable@...nel.org
---
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 6f8ec1c..fc801ba 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -181,7 +181,7 @@ static inline void native_cpuid(unsigned int *eax, unsigned int *ebx,
 				unsigned int *ecx, unsigned int *edx)
 {
 	/* ecx is often an input as well as an output. */
-	asm("cpuid"
+	asm volatile("cpuid"
 	    : "=a" (*eax),
 	      "=b" (*ebx),
 	      "=c" (*ecx),


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