KVM: VMX: Skip VMCLEAR logic during emergency reboots if CR4.VMXE=0

Bail from vmx_emergency_disable() without processing the list of loaded
VMCSes if CR4.VMXE=0, i.e. if the CPU can't be post-VMXON.  It should be
impossible for the list to have entries if VMX is already disabled, and
even if that invariant doesn't hold, VMCLEAR will #UD anyways, i.e.
processing the list is pointless even if it somehow isn't empty.

Assuming no existing KVM bugs, this should be a glorified nop.  The
primary motivation for the change is to avoid having code that looks like
it does VMCLEAR, but then skips VMXON, which is nonsensical.

Suggested-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20230721201859.2307736-20-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/vmx/vmx.c

@@ -754,12 +754,20 @@ static void vmx_emergency_disable(void)
 
 	kvm_rebooting = true;
 
+	/*
+	 * Note, CR4.VMXE can be _cleared_ in NMI context, but it can only be
+	 * set in task context.  If this races with VMX is disabled by an NMI,
+	 * VMCLEAR and VMXOFF may #UD, but KVM will eat those faults due to
+	 * kvm_rebooting set.
+	 */
+	if (!(__read_cr4() & X86_CR4_VMXE))
+		return;
+
 	list_for_each_entry(v, &per_cpu(loaded_vmcss_on_cpu, cpu),
 			    loaded_vmcss_on_cpu_link)
 		vmcs_clear(v->vmcs);
 
-	if (__read_cr4() & X86_CR4_VMXE)
-		kvm_cpu_vmxoff();
+	kvm_cpu_vmxoff();
 }
 
 static void __loaded_vmcs_clear(void *arg)