[PATCH] microcode: do not WARN_ON(cpu != 0) during resume

[Ian Campbell <ian.campbell@citrix.com>]

871b72dd "x86: microcode: use smp_call_function_single instead of
set_cpus_allowed, cleanup of synchronization logic" included:

 static int mc_sysdev_resume(struct sys_device *dev)
 {
[...]
+       /*
+        * All non-bootup cpus are still disabled,
+        * so only CPU 0 will apply ucode here.
+        *
+        * Moreover, there can be no concurrent
+        * updates from any other places at this point.
+        */
+       WARN_ON(cpu != 0);

However suspend/resume under Xen doesn't need to hot unplug all the CPUs, so we
don't; the hypervisor can manage the context save/restore for all CPUs.

It would be unnecessary to load microcode.ko in a Xen domU but if it does occur
(e.g. because a distro installs the tools by default) we would like to avoid
the warning on resume.

Since the real constraint here is that we are running on the CPU for which we
would like to load microcode (which in all practical circumstances is CPU0)
just check for that and return if we are resuming a different CPU.

There is no danger of concurrent updates, even if we ignore the fact that all
but one CPUs are unplugged on native, because sysdev_resume() is single
threaded.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: Ingo Molnar <mingo@elte.hu>
---
 arch/x86/kernel/microcode_core.c |   11 +----------
 1 files changed, 1 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kernel/microcode_core.c b/arch/x86/kernel/microcode_core.c
index 378e9a8..1153062 100644
--- a/arch/x86/kernel/microcode_core.c
+++ b/arch/x86/kernel/microcode_core.c
@@ -438,18 +438,9 @@ static int mc_sysdev_resume(struct sys_device *dev)
 	int cpu = dev->id;
 	struct ucode_cpu_info *uci = ucode_cpu_info + cpu;
 
-	if (!cpu_online(cpu))
+	if (cpu != smp_processor_id())
 		return 0;
 
-	/*
-	 * All non-bootup cpus are still disabled,
-	 * so only CPU 0 will apply ucode here.
-	 *
-	 * Moreover, there can be no concurrent
-	 * updates from any other places at this point.
-	 */
-	WARN_ON(cpu != 0);
-
 	if (uci->valid && uci->mc)
 		microcode_ops->apply_microcode(cpu);
 
-- 
1.5.6.5