x86/hyper-V: Allocate the IDT entry early in boot

Allocate the hypervisor callback IDT entry early in the boot sequence.

The previous code would allocate the entry as part of registering the handler
when the vmbus driver loaded, and this caused a problem for the IDT cleanup
that Thomas is working on for v4.15.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: apw@canonical.com
Cc: devel@linuxdriverproject.org
Cc: gregkh@linuxfoundation.org
Cc: jasowang@redhat.com
Cc: olaf@aepfle.de
Link: http://lkml.kernel.org/r/20170908231557.2419-1-kys@exchange.microsoft.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/cpu/mshyperv.c

@@ -59,8 +59,6 @@ void hyperv_vector_handler(struct pt_regs *regs)
 void hv_setup_vmbus_irq(void (*handler)(void))
 {
 	vmbus_handler = handler;
-	/* Setup the IDT for hypervisor callback */
-	alloc_intr_gate(HYPERVISOR_CALLBACK_VECTOR, hyperv_callback_vector);
 }
 
 void hv_remove_vmbus_irq(void)
@@ -251,6 +249,8 @@ static void __init ms_hyperv_init_platform(void)
 	 */
 	x86_platform.apic_post_init = hyperv_init;
 	hyperv_setup_mmu_ops();
+	/* Setup the IDT for hypervisor callback */
+	alloc_intr_gate(HYPERVISOR_CALLBACK_VECTOR, hyperv_callback_vector);
 #endif
 }