KVM: Fetch guest cr3 from hardware on demand

Instead of syncing the guest cr3 every exit, which is expensince on vmx
with ept enabled, sync it only on demand.

[sheng: fix incorrect cr3 seen by Windows XP]

Signed-off-by: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index a7b04c0..25bd1bc 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1327,6 +1327,10 @@
 {
 }
 
+static void svm_decache_cr3(struct kvm_vcpu *vcpu)
+{
+}
+
 static void svm_decache_cr4_guest_bits(struct kvm_vcpu *vcpu)
 {
 }
@@ -3871,6 +3875,7 @@
 	.get_cpl = svm_get_cpl,
 	.get_cs_db_l_bits = kvm_get_cs_db_l_bits,
 	.decache_cr0_guest_bits = svm_decache_cr0_guest_bits,
+	.decache_cr3 = svm_decache_cr3,
 	.decache_cr4_guest_bits = svm_decache_cr4_guest_bits,
 	.set_cr0 = svm_set_cr0,
 	.set_cr3 = svm_set_cr3,