xen: support GSI -> pirq remapping in PV on HVM guests

Disable pcifront when running on HVM: it is meant to be used with pv
guests that don't have PCI bus.

Use acpi_register_gsi_xen_hvm to remap GSIs into pirqs.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/pci/xen.c

@@ -14,6 +14,7 @@
 
 #include <asm/xen/hypervisor.h>
 
+#include <xen/features.h>
 #include <xen/events.h>
 #include <asm/xen/pci.h>
 
@@ -184,3 +185,18 @@ int __init pci_xen_init(void)
 #endif
 	return 0;
 }
+
+int __init pci_xen_hvm_init(void)
+{
+	if (!xen_feature(XENFEAT_hvm_pirqs))
+		return 0;
+
+#ifdef CONFIG_ACPI
+	/*
+	 * We don't want to change the actual ACPI delivery model,
+	 * just how GSIs get registered.
+	 */
+	__acpi_register_gsi = acpi_register_gsi_xen_hvm;
+#endif
+	return 0;
+}