virtio-scsi: implement BlockDevOps->drained_begin()

The virtio-scsi Host Bus Adapter provides access to devices on a SCSI
bus. Those SCSI devices typically have a BlockBackend. When the
BlockBackend enters a drained section, the SCSI device must temporarily
stop submitting new I/O requests.

Implement this behavior by temporarily stopping virtio-scsi virtqueue
processing when one of the SCSI devices enters a drained section. The
new scsi_device_drained_begin() API allows scsi-disk to message the
virtio-scsi HBA.

scsi_device_drained_begin() uses a drain counter so that multiple SCSI
devices can have overlapping drained sections. The HBA only sees one
pair of .drained_begin/end() calls.

After this commit, virtio-scsi no longer depends on hw/virtio's
ioeventfd aio_set_event_notifier(is_external=true). This commit is a
step towards removing the aio_disable_external() API.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230516190238.8401-19-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 22 insertions, 5 deletions

diff --git a/hw/scsi/scsi-disk.c b/hw/scsi/scsi-disk.c
index 97c9b1c8cd..e0d79c7966 100644
--- a/hw/scsi/scsi-disk.c
+++ b/hw/scsi/scsi-disk.c
@@ -2360,6 +2360,20 @@ static void scsi_disk_reset(DeviceState *dev)
     s->qdev.scsi_version = s->qdev.default_scsi_version;
 }
 
+static void scsi_disk_drained_begin(void *opaque)
+{
+    SCSIDiskState *s = opaque;
+
+    scsi_device_drained_begin(&s->qdev);
+}
+
+static void scsi_disk_drained_end(void *opaque)
+{
+    SCSIDiskState *s = opaque;
+
+    scsi_device_drained_end(&s->qdev);
+}
+
 static void scsi_disk_resize_cb(void *opaque)
 {
     SCSIDiskState *s = opaque;
@@ -2414,16 +2428,19 @@ static bool scsi_cd_is_medium_locked(void *opaque)
 }
 
 static const BlockDevOps scsi_disk_removable_block_ops = {
-    .change_media_cb = scsi_cd_change_media_cb,
+    .change_media_cb  = scsi_cd_change_media_cb,
+    .drained_begin    = scsi_disk_drained_begin,
+    .drained_end      = scsi_disk_drained_end,
     .eject_request_cb = scsi_cd_eject_request_cb,
-    .is_tray_open = scsi_cd_is_tray_open,
     .is_medium_locked = scsi_cd_is_medium_locked,
-
-    .resize_cb = scsi_disk_resize_cb,
+    .is_tray_open     = scsi_cd_is_tray_open,
+    .resize_cb        = scsi_disk_resize_cb,
 };
 
 static const BlockDevOps scsi_disk_block_ops = {
-    .resize_cb = scsi_disk_resize_cb,
+    .drained_begin = scsi_disk_drained_begin,
+    .drained_end   = scsi_disk_drained_end,
+    .resize_cb     = scsi_disk_resize_cb,
 };
 
 static void scsi_disk_unit_attention_reported(SCSIDevice *dev)