KVM: VMX: Drop guest CPUID check for VMXE in vmx_set_cr4()

Drop vmx_set_cr4()'s somewhat hidden guest_cpuid_has() check on VMXE now
that common x86 handles the check by incorporating VMXE into the CR4
reserved bits, i.e. in cr4_guest_rsvd_bits.  This fixes a bug where KVM
incorrectly rejects KVM_SET_SREGS with CR4.VMXE=1 if it's executed
before KVM_SET_CPUID{,2}.

Fixes: 5e1746d6 ("KVM: nVMX: Allow setting the VMXE bit in CR4")
Reported-by: Stas Sergeev <stsp@users.sourceforge.net>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20201007014417.29276-2-sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -3129,9 +3129,10 @@ int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 		 * must first be able to turn on cr4.VMXE (see handle_vmon()).
 		 * So basically the check on whether to allow nested VMX
 		 * is here.  We operate under the default treatment of SMM,
-		 * so VMX cannot be enabled under SMM.
+		 * so VMX cannot be enabled under SMM.  Note, guest CPUID is
+		 * intentionally ignored, it's handled by cr4_guest_rsvd_bits.
 		 */
-		if (!nested_vmx_allowed(vcpu) || is_smm(vcpu))
+		if (!nested || is_smm(vcpu))
 			return 1;
 	}