x86, hyperv: Handle Xen emulation of Hyper-V more gracefully

Install the Hyper-V specific interrupt handler only when needed. This would
permit us to get rid of the Xen check. Note that when the vmbus drivers invokes
the call to register its handler, we are sure to be running on Hyper-V.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Link: http://lkml.kernel.org/r/1366299886-6399-1-git-send-email-kys@microsoft.com
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/kernel/cpu/mshyperv.c

@@ -35,13 +35,6 @@ static bool __init ms_hyperv_platform(void)
 	if (!boot_cpu_has(X86_FEATURE_HYPERVISOR))
 		return false;
 
-	/*
-	 * Xen emulates Hyper-V to support enlightened Windows.
-	 * Check to see first if we are on a Xen Hypervisor.
-	 */
-	if (xen_cpuid_base())
-		return false;
-
 	cpuid(HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS,
 	      &eax, &hyp_signature[0], &hyp_signature[1], &hyp_signature[2]);
 
@@ -82,12 +75,6 @@ static void __init ms_hyperv_init_platform(void)
 
 	if (ms_hyperv.features & HV_X64_MSR_TIME_REF_COUNT_AVAILABLE)
 		clocksource_register_hz(&hyperv_cs, NSEC_PER_SEC/100);
-#if IS_ENABLED(CONFIG_HYPERV)
-	/*
-	 * Setup the IDT for hypervisor callback.
-	 */
-	alloc_intr_gate(HYPERVISOR_CALLBACK_VECTOR, hyperv_callback_vector);
-#endif
 }
 
 const __refconst struct hypervisor_x86 x86_hyper_ms_hyperv = {
@@ -103,6 +90,11 @@ static irq_handler_t vmbus_isr;
 
 void hv_register_vmbus_handler(int irq, irq_handler_t handler)
 {
+	/*
+	 * Setup the IDT for hypervisor callback.
+	 */
+	alloc_intr_gate(HYPERVISOR_CALLBACK_VECTOR, hyperv_callback_vector);
+
 	vmbus_irq = irq;
 	vmbus_isr = handler;
 }