xen: mask extended topology info in cpuid

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@citrix.com>
Cc: Stable Kernel <stable@kernel.org>

1 files changed, 10 insertions, 1 deletions

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 23a4d80fb39e..dfbf70e65860 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -178,6 +178,7 @@ static __read_mostly unsigned int cpuid_leaf1_ecx_mask = ~0;
 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, unsigned int *bx,
          * 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, unsigned int *bx,
                   "=d" (*dx)
                 : "0" (*ax), "2" (*cx));
 
+        *bx &= maskebx;
         *cx &= maskecx;
         *dx &= maskedx;
 }