[Qemu-devel] [PATCH v4 11/11] virtio-blk: add x-data-plane=on|off performance feature

[Stefan Hajnoczi <stefanha@redhat.com>]

The virtio-blk-data-plane feature is easy to integrate into
hw/virtio-blk.c.  The data plane can be started and stopped similar to
vhost-net.

Users can take advantage of the virtio-blk-data-plane feature using the
new -device virtio-blk-pci,x-data-plane=on property.

The x-data-plane name was chosen because at this stage the feature is
experimental and likely to see changes in the future.

If the VM configuration does not support virtio-blk-data-plane an error
message is printed.  Although we could fall back to regular virtio-blk,
I prefer the explicit approach since it prompts the user to fix their
configuration if they want the performance benefit of
virtio-blk-data-plane.

Limitations:
 * Only format=raw is supported
 * Live migration is not supported
 * Block jobs, hot unplug, and other operations fail with -EBUSY
 * I/O throttling limits are ignored
 * Only Linux hosts are supported due to Linux AIO usage

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 hw/virtio-blk.c | 59 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 hw/virtio-blk.h |  1 +
 hw/virtio-pci.c |  3 +++
 3 files changed, 62 insertions(+), 1 deletion(-)

diff --git a/hw/virtio-blk.c b/hw/virtio-blk.c
index e25cc96..7f6004e 100644
--- a/hw/virtio-blk.c
+++ b/hw/virtio-blk.c
@@ -17,6 +17,8 @@
 #include "hw/block-common.h"
 #include "blockdev.h"
 #include "virtio-blk.h"
+#include "hw/dataplane/virtio-blk.h"
+#include "migration.h"
 #include "scsi-defs.h"
 #ifdef __linux__
 # include <scsi/sg.h>
@@ -33,6 +35,8 @@ typedef struct VirtIOBlock
     VirtIOBlkConf *blk;
     unsigned short sector_mask;
     DeviceState *qdev;
+    VirtIOBlockDataPlane *dataplane;
+    Error *migration_blocker;
 } VirtIOBlock;
 
 static VirtIOBlock *to_virtio_blk(VirtIODevice *vdev)
@@ -407,6 +411,14 @@ static void virtio_blk_handle_output(VirtIODevice *vdev, VirtQueue *vq)
         .num_writes = 0,
     };
 
+    /* Some guests kick before setting VIRTIO_CONFIG_S_DRIVER_OK so start
+     * dataplane here instead of waiting for .set_status().
+     */
+    if (s->dataplane) {
+        virtio_blk_data_plane_start(s->dataplane);
+        return;
+    }
+
     while ((req = virtio_blk_get_request(s))) {
         virtio_blk_handle_request(req, &mrb);
     }
@@ -446,8 +458,13 @@ static void virtio_blk_dma_restart_cb(void *opaque, int running,
 {
     VirtIOBlock *s = opaque;
 
-    if (!running)
+    if (!running) {
+        /* qemu_drain_all() doesn't know about data plane, quiesce here */
+        if (s->dataplane) {
+            virtio_blk_data_plane_drain(s->dataplane);
+        }
         return;
+    }
 
     if (!s->bh) {
         s->bh = qemu_bh_new(virtio_blk_dma_restart_bh, s);
@@ -538,6 +555,10 @@ static void virtio_blk_set_status(VirtIODevice *vdev, uint8_t status)
     VirtIOBlock *s = to_virtio_blk(vdev);
     uint32_t features;
 
+    if (s->dataplane && !(status & VIRTIO_CONFIG_S_DRIVER)) {
+        virtio_blk_data_plane_stop(s->dataplane);
+    }
+
     if (!(status & VIRTIO_CONFIG_S_DRIVER_OK)) {
         return;
     }
@@ -604,6 +625,7 @@ VirtIODevice *virtio_blk_init(DeviceState *dev, VirtIOBlkConf *blk)
 {
     VirtIOBlock *s;
     static int virtio_blk_id;
+    int fd = -1;
 
     if (!blk->conf.bs) {
         error_report("drive property not set");
@@ -619,6 +641,21 @@ VirtIODevice *virtio_blk_init(DeviceState *dev, VirtIOBlkConf *blk)
         return NULL;
     }
 
+    if (blk->data_plane) {
+        if (blk->scsi) {
+            error_report("device is incompatible with x-data-plane, "
+                         "use scsi=off");
+            return NULL;
+        }
+
+        fd = raw_get_aio_fd(blk->conf.bs);
+        if (fd < 0) {
+            error_report("drive is incompatible with x-data-plane, "
+                         "use format=raw,cache=none,aio=native");
+            return NULL;
+        }
+    }
+
     s = (VirtIOBlock *)virtio_common_init("virtio-blk", VIRTIO_ID_BLOCK,
                                           sizeof(struct virtio_blk_config),
                                           sizeof(VirtIOBlock));
@@ -636,6 +673,17 @@ VirtIODevice *virtio_blk_init(DeviceState *dev, VirtIOBlkConf *blk)
 
     s->vq = virtio_add_queue(&s->vdev, 128, virtio_blk_handle_output);
 
+    if (fd >= 0) {
+        s->dataplane = virtio_blk_data_plane_create(&s->vdev, fd);
+
+        /* Prevent block operations that conflict with data plane thread */
+        bdrv_set_in_use(s->bs, 1);
+
+        error_setg(&s->migration_blocker,
+                   "x-data-plane does not support migration");
+        migrate_add_blocker(s->migration_blocker);
+    }
+
     qemu_add_vm_change_state_handler(virtio_blk_dma_restart_cb, s);
     s->qdev = dev;
     register_savevm(dev, "virtio-blk", virtio_blk_id++, 2,
@@ -652,6 +700,15 @@ VirtIODevice *virtio_blk_init(DeviceState *dev, VirtIOBlkConf *blk)
 void virtio_blk_exit(VirtIODevice *vdev)
 {
     VirtIOBlock *s = to_virtio_blk(vdev);
+
+    if (s->dataplane) {
+        migrate_del_blocker(s->migration_blocker);
+        error_free(s->migration_blocker);
+        bdrv_set_in_use(s->bs, 0);
+        virtio_blk_data_plane_destroy(s->dataplane);
+        s->dataplane = NULL;
+    }
+
     unregister_savevm(s->qdev, "virtio-blk", s);
     blockdev_mark_auto_del(s->bs);
     virtio_cleanup(vdev);
diff --git a/hw/virtio-blk.h b/hw/virtio-blk.h
index f0740d0..53d7971 100644
--- a/hw/virtio-blk.h
+++ b/hw/virtio-blk.h
@@ -105,6 +105,7 @@ struct VirtIOBlkConf
     char *serial;
     uint32_t scsi;
     uint32_t config_wce;
+    uint32_t data_plane;
 };
 
 #define DEFINE_VIRTIO_BLK_FEATURES(_state, _field) \
diff --git a/hw/virtio-pci.c b/hw/virtio-pci.c
index 71f4fb5..32cc910 100644
--- a/hw/virtio-pci.c
+++ b/hw/virtio-pci.c
@@ -897,6 +897,9 @@ static Property virtio_blk_properties[] = {
 #endif
     DEFINE_PROP_BIT("config-wce", VirtIOPCIProxy, blk.config_wce, 0, true),
     DEFINE_PROP_BIT("ioeventfd", VirtIOPCIProxy, flags, VIRTIO_PCI_FLAG_USE_IOEVENTFD_BIT, true),
+#ifdef CONFIG_VIRTIO_BLK_DATA_PLANE
+    DEFINE_PROP_BIT("x-data-plane", VirtIOPCIProxy, blk.data_plane, 0, false),
+#endif
     DEFINE_PROP_UINT32("vectors", VirtIOPCIProxy, nvectors, 2),
     DEFINE_VIRTIO_BLK_FEATURES(VirtIOPCIProxy, host_features),
     DEFINE_PROP_END_OF_LIST(),
-- 
1.8.0