[61/99] xen: mask extended topology info in cpuid

[Greg KH]

2.6.31-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>

commit 82d6469916c6fcfa345636a49004c9d1753905d1 upstream.

A Xen guest never needs to know about extended topology, and knowing
would just confuse it.

This patch just zeros ebx in leaf 0xb which indicates no topology info,
preventing a crash under Xen on cpus which support this leaf.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>

---
 arch/x86/xen/enlighten.c |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -178,6 +178,7 @@ static __read_mostly unsigned int cpuid_
 static void xen_cpuid(unsigned int *ax, unsigned int *bx,
 		      unsigned int *cx, unsigned int *dx)
 {
+	unsigned maskebx = ~0;
 	unsigned maskecx = ~0;
 	unsigned maskedx = ~0;
 
@@ -185,9 +186,16 @@ static void xen_cpuid(unsigned int *ax, 
 	 * Mask out inconvenient features, to try and disable as many
 	 * unsupported kernel subsystems as possible.
 	 */
-	if (*ax == 1) {
+	switch (*ax) {
+	case 1:
 		maskecx = cpuid_leaf1_ecx_mask;
 		maskedx = cpuid_leaf1_edx_mask;
+		break;
+
+	case 0xb:
+		/* Suppress extended topology stuff */
+		maskebx = 0;
+		break;
 	}
 
 	asm(XEN_EMULATE_PREFIX "cpuid"
@@ -197,6 +205,7 @@ static void xen_cpuid(unsigned int *ax, 
 		  "=d" (*dx)
 		: "0" (*ax), "2" (*cx));
 
+	*bx &= maskebx;
 	*cx &= maskecx;
 	*dx &= maskedx;
 }


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