x86, microcode: Don't initialize microcode code on paravirt

commit a18a0f68 upstream.

Paravirtual guests are not expected to load microcode into processors
and therefore it is not necessary to initialize microcode loading
logic.

In fact, under certain circumstances initializing this logic may cause
the guest to crash. Specifically, 32-bit kernels use __pa_nodebug()
macro which does not work in Xen (the code path that leads to this macro
happens during resume when we call mc_bp_resume()->load_ucode_ap()
->check_loader_disabled_ap())
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: http://lkml.kernel.org/r/1417469264-31470-1-git-send-email-boris.ostrovsky@oracle.comSigned-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/kernel/cpu/microcode/core.c

@@ -559,7 +559,7 @@ static int __init microcode_init(void)
 	struct cpuinfo_x86 *c = &cpu_data(0);
 	int error;
 
-	if (dis_ucode_ldr)
+	if (paravirt_enabled() || dis_ucode_ldr)
 		return 0;
 
 	if (c->x86_vendor == X86_VENDOR_INTEL)