[Qemu-devel] [PATCH 9/9] dataplane: use a QContext event loop in place of custom thread

[Michael Roth]

virtio-blk dataplane currently creates/manages it's own thread to
offload work to a separate event loop.

This patch insteads allows us to specify a QContext-based event loop by
adding a "context" property for virtio-blk we can use like so:

  qemu ... \
    -object glib-qcontext,id=ctx0,threaded=yes
    -drive file=file.raw,id=drive0,aio=native,cache=none \
    -device virtio-blk,drive=drive0,scsi=off,x-data-plane=on,context=ctx0

virtio-blk dataplane then simply attachs/detaches it's AioContext to the
ctx0 event loop on start/stop.

This also makes available the option to drive a virtio-blk dataplane via
the default main loop:

  qemu ... \
    -drive file=file.raw,id=drive0,aio=native,cache=none \
    -device virtio-blk,drive=drive0,scsi=off,x-data-plane=on,context=main

This doesn't do much in and of itself, but helps to demonstrate how we
might model a general mechanism to offload device workloads to separate
threads.

Signed-off-by: Michael Roth <address@hidden>
---
 hw/block/dataplane/virtio-blk.c |   46 ++++++++++++---------------------------
 include/hw/virtio/virtio-blk.h  |    7 ++++--
 2 files changed, 19 insertions(+), 34 deletions(-)

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index 0356665..08ea10f 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -24,6 +24,8 @@
 #include "virtio-blk.h"
 #include "block/aio.h"
 #include "hw/virtio/virtio-bus.h"
+#include "qcontext/qcontext.h"
+#include "qcontext/glib-qcontext.h"
 
 enum {
     SEG_MAX = 126,                  /* maximum number of I/O segments */
@@ -60,6 +62,7 @@ struct VirtIOBlockDataPlane {
      * use it).
      */
     AioContext *ctx;
+    QContext *qctx;
     EventNotifier io_notifier;      /* Linux AIO completion */
     EventNotifier host_notifier;    /* doorbell */
 
@@ -375,26 +378,6 @@ static void handle_io(EventNotifier *e)
     }
 }
 
-static void *data_plane_thread(void *opaque)
-{
-    VirtIOBlockDataPlane *s = opaque;
-
-    do {
-        aio_poll(s->ctx, true);
-    } while (!s->stopping || s->num_reqs > 0);
-    return NULL;
-}
-
-static void start_data_plane_bh(void *opaque)
-{
-    VirtIOBlockDataPlane *s = opaque;
-
-    qemu_bh_delete(s->start_bh);
-    s->start_bh = NULL;
-    qemu_thread_create(&s->thread, data_plane_thread,
-                       s, QEMU_THREAD_JOINABLE);
-}
-
 bool virtio_blk_data_plane_create(VirtIODevice *vdev, VirtIOBlkConf *blk,
                                   VirtIOBlockDataPlane **dataplane)
 {
@@ -460,6 +443,7 @@ void virtio_blk_data_plane_start(VirtIOBlockDataPlane *s)
     VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(qbus);
     VirtQueue *vq;
     int i;
+    Error *err = NULL;
 
     if (s->started) {
         return;
@@ -502,9 +486,16 @@ void virtio_blk_data_plane_start(VirtIOBlockDataPlane *s)
     /* Kick right away to begin processing requests already in vring */
     event_notifier_set(virtio_queue_get_host_notifier(vq));
 
-    /* Spawn thread in BH so it inherits iothread cpusets */
-    s->start_bh = qemu_bh_new(start_data_plane_bh, s);
-    qemu_bh_schedule(s->start_bh);
+    /* use QEMU main loop/context by default */
+    if (!s->blk->context) {
+        s->blk->context = g_strdup("main");
+    }
+    s->qctx = qcontext_find_by_name(s->blk->context, &err);
+    if (err) {
+        fprintf(stderr, "virtio-blk failed to start: %s", 
error_get_pretty(err));
+        exit(1);
+    }
+    aio_context_attach(s->ctx, s->qctx);
 }
 
 void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
@@ -517,15 +508,6 @@ void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
     s->stopping = true;
     trace_virtio_blk_data_plane_stop(s);
 
-    /* Stop thread or cancel pending thread creation BH */
-    if (s->start_bh) {
-        qemu_bh_delete(s->start_bh);
-        s->start_bh = NULL;
-    } else {
-        aio_notify(s->ctx);
-        qemu_thread_join(&s->thread);
-    }
-
     aio_set_event_notifier(s->ctx, &s->io_notifier, NULL, NULL);
     ioq_cleanup(&s->ioqueue);
 
diff --git a/include/hw/virtio/virtio-blk.h b/include/hw/virtio/virtio-blk.h
index fc71853..c5514a4 100644
--- a/include/hw/virtio/virtio-blk.h
+++ b/include/hw/virtio/virtio-blk.h
@@ -110,6 +110,7 @@ struct VirtIOBlkConf
     uint32_t scsi;
     uint32_t config_wce;
     uint32_t data_plane;
+    char *context;
 };
 
 struct VirtIOBlockDataPlane;
@@ -138,13 +139,15 @@ typedef struct VirtIOBlock {
         DEFINE_BLOCK_CHS_PROPERTIES(_state, _field.conf),                     \
         DEFINE_PROP_STRING("serial", _state, _field.serial),                  \
         DEFINE_PROP_BIT("config-wce", _state, _field.config_wce, 0, true),    \
-        DEFINE_PROP_BIT("scsi", _state, _field.scsi, 0, true)
+        DEFINE_PROP_BIT("scsi", _state, _field.scsi, 0, true),                \
+        DEFINE_PROP_STRING("context", _state, _field.context)
 #else
 #define DEFINE_VIRTIO_BLK_PROPERTIES(_state, _field)                          \
         DEFINE_BLOCK_PROPERTIES(_state, _field.conf),                         \
         DEFINE_BLOCK_CHS_PROPERTIES(_state, _field.conf),                     \
         DEFINE_PROP_STRING("serial", _state, _field.serial),                  \
-        DEFINE_PROP_BIT("config-wce", _state, _field.config_wce, 0, true)
+        DEFINE_PROP_BIT("config-wce", _state, _field.config_wce, 0, true),    \
+        DEFINE_PROP_STRING("context", _state, _field.context)
 #endif /* __linux__ */
 
 void virtio_blk_set_conf(DeviceState *dev, VirtIOBlkConf *blk);
-- 
1.7.9.5