hw/xen: Add evtchn operations to allow redirection to internal emulation

The existing implementation calling into the real libxenevtchn moves to
a new file hw/xen/xen-operations.c, and is called via a function table
which in a subsequent commit will also be able to invoke the emulated
event channel support.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>

include/hw/xen/xen_backend_ops.h

@@ -0,0 +1,118 @@
+/*
+ * QEMU Xen backend support
+ *
+ * Copyright © 2022 Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ *
+ * Authors: David Woodhouse <dwmw2@infradead.org>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#ifndef QEMU_XEN_BACKEND_OPS_H
+#define QEMU_XEN_BACKEND_OPS_H
+
+/*
+ * For the time being, these operations map fairly closely to the API of
+ * the actual Xen libraries, e.g. libxenevtchn. As we complete the migration
+ * from XenLegacyDevice back ends to the new XenDevice model, they may
+ * evolve to slightly higher-level APIs.
+ *
+ * The internal emulations do not emulate the Xen APIs entirely faithfully;
+ * only enough to be used by the Xen backend devices. For example, only one
+ * event channel can be bound to each handle, since that's sufficient for
+ * the device support (only the true Xen HVM backend uses more). And the
+ * behaviour of unmask() and pending() is different too because the device
+ * backends don't care.
+ */
+
+typedef struct xenevtchn_handle xenevtchn_handle;
+typedef int xenevtchn_port_or_error_t;
+typedef uint32_t evtchn_port_t;
+
+struct evtchn_backend_ops {
+    xenevtchn_handle *(*open)(void);
+    int (*bind_interdomain)(xenevtchn_handle *xc, uint32_t domid,
+                            evtchn_port_t guest_port);
+    int (*unbind)(xenevtchn_handle *xc, evtchn_port_t port);
+    int (*close)(struct xenevtchn_handle *xc);
+    int (*get_fd)(struct xenevtchn_handle *xc);
+    int (*notify)(struct xenevtchn_handle *xc, evtchn_port_t port);
+    int (*unmask)(struct xenevtchn_handle *xc, evtchn_port_t port);
+    int (*pending)(struct xenevtchn_handle *xc);
+};
+
+extern struct evtchn_backend_ops *xen_evtchn_ops;
+
+static inline xenevtchn_handle *qemu_xen_evtchn_open(void)
+{
+    if (!xen_evtchn_ops) {
+        return NULL;
+    }
+    return xen_evtchn_ops->open();
+}
+
+static inline int qemu_xen_evtchn_bind_interdomain(xenevtchn_handle *xc,
+                                                   uint32_t domid,
+                                                   evtchn_port_t guest_port)
+{
+    if (!xen_evtchn_ops) {
+        return -ENOSYS;
+    }
+    return xen_evtchn_ops->bind_interdomain(xc, domid, guest_port);
+}
+
+static inline int qemu_xen_evtchn_unbind(xenevtchn_handle *xc,
+                                         evtchn_port_t port)
+{
+    if (!xen_evtchn_ops) {
+        return -ENOSYS;
+    }
+    return xen_evtchn_ops->unbind(xc, port);
+}
+
+static inline int qemu_xen_evtchn_close(xenevtchn_handle *xc)
+{
+    if (!xen_evtchn_ops) {
+        return -ENOSYS;
+    }
+    return xen_evtchn_ops->close(xc);
+}
+
+static inline int qemu_xen_evtchn_fd(xenevtchn_handle *xc)
+{
+    if (!xen_evtchn_ops) {
+        return -ENOSYS;
+    }
+    return xen_evtchn_ops->get_fd(xc);
+}
+
+static inline int qemu_xen_evtchn_notify(xenevtchn_handle *xc,
+                                         evtchn_port_t port)
+{
+    if (!xen_evtchn_ops) {
+        return -ENOSYS;
+    }
+    return xen_evtchn_ops->notify(xc, port);
+}
+
+static inline int qemu_xen_evtchn_unmask(xenevtchn_handle *xc,
+                                         evtchn_port_t port)
+{
+    if (!xen_evtchn_ops) {
+        return -ENOSYS;
+    }
+    return xen_evtchn_ops->unmask(xc, port);
+}
+
+static inline int qemu_xen_evtchn_pending(xenevtchn_handle *xc)
+{
+    if (!xen_evtchn_ops) {
+        return -ENOSYS;
+    }
+    return xen_evtchn_ops->pending(xc);
+}
+
+void setup_xen_backend_ops(void);
+
+#endif /* QEMU_XEN_BACKEND_OPS_H */