xen-bus / xen-block: add support for event channel polling

This patch introduces a poll callback for event channel fd-s and uses
this to invoke the channel callback function.

To properly support polling, it is necessary for the event channel callback
function to return a boolean saying whether it has done any useful work or
not. Thus xen_block_dataplane_event() is modified to directly invoke
xen_block_handle_requests() and the latter only returns true if it actually
processes any requests. This also means that the call to qemu_bh_schedule()
is moved into xen_block_complete_aio(), which is more intuitive since the
only reason for doing a deferred poll of the shared ring should be because
there were previously insufficient resources to fully complete a previous
poll.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20190408151617.13025-4-paul.durrant@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

hw/block/dataplane/xen-block.c

@@ -318,7 +318,9 @@ static void xen_block_complete_aio(void *opaque, int ret)
     }
     xen_block_release_request(request);
 
-    qemu_bh_schedule(dataplane->bh);
+    if (dataplane->more_work) {
+        qemu_bh_schedule(dataplane->bh);
+    }
 
 done:
     aio_context_release(dataplane->ctx);
@@ -515,12 +517,13 @@ static int xen_block_get_request(XenBlockDataPlane *dataplane,
  */
 #define IO_PLUG_THRESHOLD 1
 
-static void xen_block_handle_requests(XenBlockDataPlane *dataplane)
+static bool xen_block_handle_requests(XenBlockDataPlane *dataplane)
 {
     RING_IDX rc, rp;
     XenBlockRequest *request;
     int inflight_atstart = dataplane->requests_inflight;
     int batched = 0;
+    bool done_something = false;
 
     dataplane->more_work = 0;
 
@@ -552,6 +555,7 @@ static void xen_block_handle_requests(XenBlockDataPlane *dataplane)
         }
         xen_block_get_request(dataplane, request, rc);
         dataplane->rings.common.req_cons = ++rc;
+        done_something = true;
 
         /* parse them */
         if (xen_block_parse_request(request) != 0) {
@@ -603,10 +607,7 @@ static void xen_block_handle_requests(XenBlockDataPlane *dataplane)
         blk_io_unplug(dataplane->blk);
     }
 
-    if (dataplane->more_work &&
-        dataplane->requests_inflight < dataplane->max_requests) {
-        qemu_bh_schedule(dataplane->bh);
-    }
+    return done_something;
 }
 
 static void xen_block_dataplane_bh(void *opaque)
@@ -618,11 +619,11 @@ static void xen_block_dataplane_bh(void *opaque)
     aio_context_release(dataplane->ctx);
 }
 
-static void xen_block_dataplane_event(void *opaque)
+static bool xen_block_dataplane_event(void *opaque)
 {
     XenBlockDataPlane *dataplane = opaque;
 
-    qemu_bh_schedule(dataplane->bh);
+    return xen_block_handle_requests(dataplane);
 }
 
 XenBlockDataPlane *xen_block_dataplane_create(XenDevice *xendev,

hw/xen/xen-bus.c

@@ -932,13 +932,20 @@ struct XenEventChannel {
     void *opaque;
 };
 
+static bool xen_device_poll(void *opaque)
+{
+    XenEventChannel *channel = opaque;
+
+    return channel->handler(channel->opaque);
+}
+
 static void xen_device_event(void *opaque)
 {
     XenEventChannel *channel = opaque;
     unsigned long port = xenevtchn_pending(channel->xeh);
 
     if (port == channel->local_port) {
-        channel->handler(channel->opaque);
+        xen_device_poll(channel);
 
         xenevtchn_unmask(channel->xeh, port);
     }
@@ -973,7 +980,7 @@ XenEventChannel *xen_device_bind_event_channel(XenDevice *xendev,
 
     channel->ctx = ctx;
     aio_set_fd_handler(channel->ctx, xenevtchn_fd(channel->xeh), true,
-                       xen_device_event, NULL, NULL, channel);
+                       xen_device_event, NULL, xen_device_poll, channel);
 
     QLIST_INSERT_HEAD(&xendev->event_channels, channel, list);
 

include/hw/xen/xen-bus.h

@@ -119,7 +119,7 @@ void xen_device_copy_grant_refs(XenDevice *xendev, bool to_domain,
                                 XenDeviceGrantCopySegment segs[],
                                 unsigned int nr_segs, Error **errp);
 
-typedef void (*XenEventHandler)(void *opaque);
+typedef bool (*XenEventHandler)(void *opaque);
 
 XenEventChannel *xen_device_bind_event_channel(XenDevice *xendev,
                                                AioContext *ctx,