x86, microcode: Don't request microcode from userspace unnecessarily

Requesting the microcode from userspace *every time* when onlining CPUs
(during a CPU hotplug operation) is unnecessary. Thus, ensure that
once the kernel gets the microcode after booting, it is not freed nor
invalidated when a CPU goes offline, so that it can be reused when that
CPU comes back online, without requesting userspace for it again. As a
result, the CPU hotplug operations become faster as well.

Signed-off-by: Srivatsa S. Bhat <[e-mail]>
Acked-by: Rafael J. Wysocki <[e-mail]>
Acked-by: Andreas Herrmann <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Borislav Petkov <[e-mail]>

arch/x86/kernel/microcode_core.c

···
 		sysfs_remove_group(&sys_dev->kobj, &mc_attr_group);
 		pr_debug("CPU%d removed\n", cpu);
 		break;
-	case CPU_DEAD:
+
+	/*
+	 * When a CPU goes offline, don't free up or invalidate the copy of
+	 * the microcode in kernel memory, so that we can reuse it when the
+	 * CPU comes back online without unnecessarily requesting the userspace
+	 * for it again.
+	 */
 	case CPU_UP_CANCELED_FROZEN:
 		/* The CPU refused to come up during a system resume */
 		microcode_fini_cpu(cpu);