[Xen-devel] [PATCH 5/6] libxl: events: Deregister evtchn fd when not needed

[Ian Jackson <ian.jackson@xxxxxxxxxxxxx>]

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@xxxxxxxxxxxxx>
---
 tools/libxl/libxl.c          |    4 +---
 tools/libxl/libxl_event.c    |   27 +++++++++++++++++++--------
 tools/libxl/libxl_internal.h |    1 +
 3 files changed, 21 insertions(+), 11 deletions(-)

diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index 785253d..e0db4eb 100644
--- a/tools/libxl/libxl.c
+++ b/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: */
diff --git a/tools/libxl/libxl_event.c b/tools/libxl/libxl_event.c
index da0a20e..716f318 100644
--- a/tools/libxl/libxl_event.c
+++ b/tools/libxl/libxl_event.c
@@ -721,7 +721,7 @@ static void evtchn_fd_callback(libxl__egc *egc, 
libxl__ev_fd *ev,
 
 int libxl__ctx_evtchn_init(libxl__gc *gc) {
     xc_evtchn *xce;
-    int rc, fd;
+    int rc;
 
     if (CTX->xce)
         return 0;
@@ -733,14 +733,10 @@ int libxl__ctx_evtchn_init(libxl__gc *gc) {
         goto out;
     }
 
-    fd = xc_evtchn_fd(xce);
-    assert(fd >= 0);
+    CTX->evtchn_fd = xc_evtchn_fd(xce);
+    assert(CTX->evtchn_fd >= 0);
 
-    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);
+    rc = libxl_fd_set_nonblock(CTX, CTX->evtchn_fd, 1);
     if (rc) goto out;
 
     CTX->xce = xce;
@@ -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 +760,13 @@ 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;
+
+    rc = libxl__ev_fd_register(gc, &CTX->evtchn_efd,
+                               evtchn_fd_callback, CTX->evtchn_fd, POLLIN);
+    if (rc) goto out;
+
     if (evev->waiting)
         return 0;
 
@@ -773,6 +782,7 @@ int libxl__ev_evtchn_wait(libxl__gc *gc, libxl__ev_evtchn 
*evev)
     return 0;
 
  out:
+    evtchn_check_fd_deregister(gc);
     return rc;
 }
 
@@ -786,6 +796,7 @@ 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);
 }
 
 /*
diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 728fe2c..481fbd5 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -359,6 +359,7 @@ struct libxl__ctx {
 
     xc_evtchn *xce; /* waiting must be done only with libxl__ev_evtchn* */
     LIBXL_LIST_HEAD(, libxl__ev_evtchn) evtchns_waiting;
+    int evtchn_fd;
     libxl__ev_fd evtchn_efd;
 
     LIBXL_TAILQ_HEAD(libxl__evgen_domain_death_list, libxl_evgen_domain_death)
-- 
1.7.10.4


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