KVM: nVMX: Allow setting the VMXE bit in CR4

This patch allows the guest to enable the VMXE bit in CR4, which is a
prerequisite to running VMXON.

Whether to allow setting the VMXE bit now depends on the architecture (svm
or vmx), so its checking has moved to kvm_x86_ops->set_cr4(). This function
now returns an int: If kvm_x86_ops->set_cr4() returns 1, __kvm_set_cr4()
will also return 1, and this will cause kvm_set_cr4() will throw a #GP.

Turning on the VMXE bit is allowed only when the nested VMX feature is
enabled, and turning it off is forbidden after a vmxon.

Signed-off-by: Nadav Har'El <nyh@il.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 506e4fe..475d1c9 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1496,11 +1496,14 @@
 	update_cr0_intercept(svm);
 }
 
-static void svm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
+static int svm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 {
 	unsigned long host_cr4_mce = read_cr4() & X86_CR4_MCE;
 	unsigned long old_cr4 = to_svm(vcpu)->vmcb->save.cr4;
 
+	if (cr4 & X86_CR4_VMXE)
+		return 1;
+
 	if (npt_enabled && ((old_cr4 ^ cr4) & X86_CR4_PGE))
 		svm_flush_tlb(vcpu);
 
@@ -1510,6 +1513,7 @@
 	cr4 |= host_cr4_mce;
 	to_svm(vcpu)->vmcb->save.cr4 = cr4;
 	mark_dirty(to_svm(vcpu)->vmcb, VMCB_CR);
+	return 0;
 }
 
 static void svm_set_segment(struct kvm_vcpu *vcpu,