[PATCH] CPU hotplug,x86 Microcode: Don't request microcode from userspace unnecessarily

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

Requesting the microcode from userspace *every time* when onlining CPUs
(during a CPU hotplug operation) is unnecessary.

So, this patch ensures 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.

Tested-by: Borislav Petkov <bp@amd64.org>
Acked-by: Borislav Petkov <bp@amd64.org>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 arch/x86/kernel/microcode_core.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kernel/microcode_core.c b/arch/x86/kernel/microcode_core.c
index f924280..3fd0189 100644
--- a/arch/x86/kernel/microcode_core.c
+++ b/arch/x86/kernel/microcode_core.c
@@ -483,7 +483,13 @@ mc_cpu_callback(struct notifier_block *nb, unsigned long action, void *hcpu)
 		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);