v4.19.13 snapshot.
diff --git a/drivers/xen/cpu_hotplug.c b/drivers/xen/cpu_hotplug.c
new file mode 100644
index 0000000..b1357aa
--- /dev/null
+++ b/drivers/xen/cpu_hotplug.c
@@ -0,0 +1,123 @@
+// SPDX-License-Identifier: GPL-2.0
+#define pr_fmt(fmt) "xen:" KBUILD_MODNAME ": " fmt
+
+#include <linux/notifier.h>
+
+#include <xen/xen.h>
+#include <xen/xenbus.h>
+
+#include <asm/xen/hypervisor.h>
+#include <asm/cpu.h>
+
+static void enable_hotplug_cpu(int cpu)
+{
+ if (!cpu_present(cpu))
+ xen_arch_register_cpu(cpu);
+
+ set_cpu_present(cpu, true);
+}
+
+static void disable_hotplug_cpu(int cpu)
+{
+ if (!cpu_is_hotpluggable(cpu))
+ return;
+ lock_device_hotplug();
+ if (cpu_online(cpu))
+ device_offline(get_cpu_device(cpu));
+ if (!cpu_online(cpu) && cpu_present(cpu)) {
+ xen_arch_unregister_cpu(cpu);
+ set_cpu_present(cpu, false);
+ }
+ unlock_device_hotplug();
+}
+
+static int vcpu_online(unsigned int cpu)
+{
+ int err;
+ char dir[16], state[16];
+
+ sprintf(dir, "cpu/%u", cpu);
+ err = xenbus_scanf(XBT_NIL, dir, "availability", "%15s", state);
+ if (err != 1) {
+ if (!xen_initial_domain())
+ pr_err("Unable to read cpu state\n");
+ return err;
+ }
+
+ if (strcmp(state, "online") == 0)
+ return 1;
+ else if (strcmp(state, "offline") == 0)
+ return 0;
+
+ pr_err("unknown state(%s) on CPU%d\n", state, cpu);
+ return -EINVAL;
+}
+static void vcpu_hotplug(unsigned int cpu)
+{
+ if (!cpu_possible(cpu))
+ return;
+
+ switch (vcpu_online(cpu)) {
+ case 1:
+ enable_hotplug_cpu(cpu);
+ break;
+ case 0:
+ disable_hotplug_cpu(cpu);
+ break;
+ default:
+ break;
+ }
+}
+
+static void handle_vcpu_hotplug_event(struct xenbus_watch *watch,
+ const char *path, const char *token)
+{
+ unsigned int cpu;
+ char *cpustr;
+
+ cpustr = strstr(path, "cpu/");
+ if (cpustr != NULL) {
+ sscanf(cpustr, "cpu/%u", &cpu);
+ vcpu_hotplug(cpu);
+ }
+}
+
+static int setup_cpu_watcher(struct notifier_block *notifier,
+ unsigned long event, void *data)
+{
+ int cpu;
+ static struct xenbus_watch cpu_watch = {
+ .node = "cpu",
+ .callback = handle_vcpu_hotplug_event};
+
+ (void)register_xenbus_watch(&cpu_watch);
+
+ for_each_possible_cpu(cpu) {
+ if (vcpu_online(cpu) == 0) {
+ (void)cpu_down(cpu);
+ set_cpu_present(cpu, false);
+ }
+ }
+
+ return NOTIFY_DONE;
+}
+
+static int __init setup_vcpu_hotplug_event(void)
+{
+ static struct notifier_block xsn_cpu = {
+ .notifier_call = setup_cpu_watcher };
+
+#ifdef CONFIG_X86
+ if (!xen_pv_domain() && !xen_pvh_domain())
+#else
+ if (!xen_domain())
+#endif
+ return -ENODEV;
+
+ register_xenstore_notifier(&xsn_cpu);
+
+ return 0;
+}
+
+arch_initcall(setup_vcpu_hotplug_event);
+