libxl: events: Deregister evtchn fd when not needed

We want to have no fd events registered when we are idle.
In this patch, deal with the evtchn fd:

 * Defer setup of the evtchn handle to the first use.
 * Defer registration of the evtchn fd; register as needed on use.
 * When cancelling an evtchn wait, or when wait setup fails, check
   whether there are now no evtchn waits and if so deregister the fd.
 * On libxl teardown, the evtchn fd should therefore be unregistered.
   assert that this is the case.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Tested-by: Ian Campbell <ian.campbell@citrix.com>
Release-Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

---
v2: Do not bother putting evtchn_fd in the ctx; instead, get it
     from xc_evtchn_fd when we need it.  (Cosmetic.)
    Do not register the evtchn fd multiple times: check it's not
     registered before we call libxl__ev_fd_register.  (Bugfix.)

tools/libxl/libxl.c

@@ -118,8 +118,6 @@ int libxl_ctx_alloc(libxl_ctx **pctx, int version,
         rc = ERROR_FAIL; goto out;
     }
 
-    rc = libxl__ctx_evtchn_init(gc);
-
     *pctx = ctx;
     return 0;
 
@@ -166,7 +164,7 @@ int libxl_ctx_free(libxl_ctx *ctx)
     for (i = 0; i < ctx->watch_nslots; i++)
         assert(!libxl__watch_slot_contents(gc, i));
     assert(!libxl__ev_fd_isregistered(&ctx->watch_efd));
-    libxl__ev_fd_deregister(gc, &ctx->evtchn_efd);
+    assert(!libxl__ev_fd_isregistered(&ctx->evtchn_efd));
     assert(!libxl__ev_fd_isregistered(&ctx->sigchld_selfpipe_efd));
 
     /* Now there should be no more events requested from the application: */

tools/libxl/libxl_event.c

@@ -739,10 +739,6 @@ int libxl__ctx_evtchn_init(libxl__gc *gc) {
     rc = libxl_fd_set_nonblock(CTX, fd, 1);
     if (rc) goto out;
 
-    rc = libxl__ev_fd_register(gc, &CTX->evtchn_efd,
-                               evtchn_fd_callback, fd, POLLIN);
-    if (rc) goto out;
-
     CTX->xce = xce;
     return 0;
 
@@ -751,6 +747,12 @@ int libxl__ctx_evtchn_init(libxl__gc *gc) {
     return rc;
 }
 
+static void evtchn_check_fd_deregister(libxl__gc *gc)
+{
+    if (CTX->xce && LIBXL_LIST_EMPTY(&CTX->evtchns_waiting))
+        libxl__ev_fd_deregister(gc, &CTX->evtchn_efd);
+}
+
 int libxl__ev_evtchn_wait(libxl__gc *gc, libxl__ev_evtchn *evev)
 {
     int r, rc;
@@ -758,6 +760,15 @@ int libxl__ev_evtchn_wait(libxl__gc *gc, libxl__ev_evtchn *evev)
     DBG("ev_evtchn=%p port=%d wait (was waiting=%d)",
         evev, evev->port, evev->waiting);
 
+    rc = libxl__ctx_evtchn_init(gc);
+    if (rc) goto out;
+
+    if (!libxl__ev_fd_isregistered(&CTX->evtchn_efd)) {
+        rc = libxl__ev_fd_register(gc, &CTX->evtchn_efd, evtchn_fd_callback,
+                                   xc_evtchn_fd(CTX->xce), POLLIN);
+        if (rc) goto out;
+    }
+
     if (evev->waiting)
         return 0;
 
@@ -773,6 +784,7 @@ int libxl__ev_evtchn_wait(libxl__gc *gc, libxl__ev_evtchn *evev)
     return 0;
 
  out:
+    evtchn_check_fd_deregister(gc);
     return rc;
 }
 
@@ -786,6 +798,7 @@ void libxl__ev_evtchn_cancel(libxl__gc *gc, libxl__ev_evtchn *evev)
 
     evev->waiting = 0;
     LIBXL_LIST_REMOVE(evev, entry);
+    evtchn_check_fd_deregister(gc);
 }
 
 /*