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, 14 insertions, 0 deletions

diff --git a/include/hw/scsi/scsi.h b/include/hw/scsi/scsi.h
index 6f23a7a73e..e2bb1a2fbf 100644
--- a/include/hw/scsi/scsi.h
+++ b/include/hw/scsi/scsi.h
@@ -133,6 +133,16 @@ struct SCSIBusInfo {
     void (*save_request)(QEMUFile *f, SCSIRequest *req);
     void *(*load_request)(QEMUFile *f, SCSIRequest *req);
     void (*free_request)(SCSIBus *bus, void *priv);
+
+    /*
+     * Temporarily stop submitting new requests between drained_begin() and
+     * drained_end(). Called from the main loop thread with the BQL held.
+     *
+     * Implement these callbacks if request processing is triggered by a file
+     * descriptor like an EventNotifier. Otherwise set them to NULL.
+     */
+    void (*drained_begin)(SCSIBus *bus);
+    void (*drained_end)(SCSIBus *bus);
 };
 
 #define TYPE_SCSI_BUS "SCSI"
@@ -144,6 +154,8 @@ struct SCSIBus {
 
     SCSISense unit_attention;
     const SCSIBusInfo *info;
+
+    int drain_count; /* protected by BQL */
 };
 
 /**
@@ -213,6 +225,8 @@ void scsi_req_cancel_complete(SCSIRequest *req);
 void scsi_req_cancel(SCSIRequest *req);
 void scsi_req_cancel_async(SCSIRequest *req, Notifier *notifier);
 void scsi_req_retry(SCSIRequest *req);
+void scsi_device_drained_begin(SCSIDevice *sdev);
+void scsi_device_drained_end(SCSIDevice *sdev);
 void scsi_device_purge_requests(SCSIDevice *sdev, SCSISense sense);
 void scsi_device_set_ua(SCSIDevice *sdev, SCSISense sense);
 void scsi_device_report_change(SCSIDevice *dev, SCSISense sense);