xen/pvh: Piggyback on PVHVM for event channels (v2)

PVH is a PV guest with a twist - there are certain things
that work in it like HVM and some like PV. There is
a similar mode - PVHVM where we run in HVM mode with
PV code enabled - and this patch explores that.

The most notable PV interfaces are the XenBus and event channels.

We will piggyback on how the event channel mechanism is
used in PVHVM - that is we want the normal native IRQ mechanism
and we will install a vector (hvm callback) for which we
will call the event channel mechanism.

This means that from a pvops perspective, we can use
native_irq_ops instead of the Xen PV specific. Albeit in the
future we could support pirq_eoi_map. But that is
a feature request that can be shared with PVHVM.

Signed-off-by: Mukesh Rathor <mukesh.rathor@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

drivers/xen/events/events_base.c

@@ -1685,8 +1685,15 @@ void __init xen_init_IRQ(void)
 	pirq_needs_eoi = pirq_needs_eoi_flag;
 
 #ifdef CONFIG_X86
-	if (xen_hvm_domain()) {
+	if (xen_pv_domain()) {
+		irq_ctx_init(smp_processor_id());
+		if (xen_initial_domain())
+			pci_xen_initial_domain();
+	}
+	if (xen_feature(XENFEAT_hvm_callback_vector))
 		xen_callback_vector();
+
+	if (xen_hvm_domain()) {
 		native_init_IRQ();
 		/* pci_xen_hvm_init must be called after native_init_IRQ so that
 		 * __acpi_register_gsi can point at the right function */
@@ -1695,13 +1702,10 @@ void __init xen_init_IRQ(void)
 		int rc;
 		struct physdev_pirq_eoi_gmfn eoi_gmfn;
 
-		irq_ctx_init(smp_processor_id());
-		if (xen_initial_domain())
-			pci_xen_initial_domain();
-
 		pirq_eoi_map = (void *)__get_free_page(GFP_KERNEL|__GFP_ZERO);
 		eoi_gmfn.gmfn = virt_to_mfn(pirq_eoi_map);
 		rc = HYPERVISOR_physdev_op(PHYSDEVOP_pirq_eoi_gmfn_v2, &eoi_gmfn);
+		/* TODO: No PVH support for PIRQ EOI */
 		if (rc != 0) {
 			free_page((unsigned long) pirq_eoi_map);
 			pirq_eoi_map = NULL;