[PATCH 2/2] block/monitor: add drive_insert HMP command

[mr-083 <matthieu@minio.io>]

Add a drive_insert HMP command that reconnects a host block device file
to an existing guest device whose backing store was previously removed
with drive_del.

After drive_del, the BlockBackend remains attached to the guest device
but has no BlockDriverState (shown as "[not inserted]" in info block).
drive_insert opens the specified file, finds the device's BlockBackend
by iterating all backends and matching the attached device ID, then
calls blk_insert_bs() to reconnect the backing store.

This complements drive_del for non-removable devices (such as NVMe
namespaces) where blockdev-change-medium cannot be used. Combined with
PCIe AER Surprise Down error injection to trigger a controller reset,
this enables complete NVMe disk hot-swap simulation where the guest
sees the same device names throughout.

Example usage:
  drive_del drv0             # remove backing store
  drive_insert ns0 disk.qcow2  # reconnect backing
  pcie_aer_inject_error rp0 SDN  # trigger controller reset

Signed-off-by: Matthieu Receveur <matthieu@min.io>
---
 block/monitor/block-hmp-cmds.c | 59 ++++++++++++++++++++++++++++++++++
 hmp-commands.hx                | 18 +++++++++++
 include/block/block-hmp-cmds.h |  1 +
 3 files changed, 78 insertions(+)

diff --git a/block/monitor/block-hmp-cmds.c b/block/monitor/block-hmp-cmds.c
index 1fd28d59eb..77e9662ead 100644
--- a/block/monitor/block-hmp-cmds.c
+++ b/block/monitor/block-hmp-cmds.c
@@ -38,7 +38,9 @@
 #include "qemu/osdep.h"
 #include "hw/core/boards.h"
 #include "system/block-backend.h"
+#include "system/block-backend-global-state.h"
 #include "system/blockdev.h"
+#include "block/block-global-state.h"
 #include "qapi/qapi-commands-block.h"
 #include "qapi/qapi-commands-block-export.h"
 #include "qobject/qdict.h"
@@ -195,6 +197,63 @@ unlock:
     hmp_handle_error(mon, err);
 }
 
+void hmp_drive_insert(Monitor *mon, const QDict *qdict)
+{
+    const char *id = qdict_get_str(qdict, "id");
+    const char *filename = qdict_get_str(qdict, "filename");
+    BlockBackend *blk = NULL;
+    BlockBackend *iter;
+    BlockDriverState *bs;
+    Error *err = NULL;
+
+    GLOBAL_STATE_CODE();
+
+    /*
+     * After drive_del, the BlockBackend is removed from the monitor name
+     * registry but still attached to the device. Find it by iterating all
+     * BlockBackends and matching by the device ID shown in "info block".
+     */
+    for (iter = blk_all_next(NULL); iter; iter = blk_all_next(iter)) {
+        DeviceState *dev = blk_get_attached_dev(iter);
+        if (dev && dev->id && strcmp(dev->id, id) == 0) {
+            blk = iter;
+            break;
+        }
+    }
+
+    if (!blk) {
+        /* Fallback: try by block backend name */
+        blk = blk_by_name(id);
+    }
+
+    if (!blk) {
+        error_setg(&err, "Device '%s' not found", id);
+        goto out;
+    }
+
+    if (blk_bs(blk)) {
+        error_setg(&err, "Device '%s' already has a medium inserted", id);
+        goto out;
+    }
+
+    bs = bdrv_open(filename, NULL, NULL, BDRV_O_RDWR, &err);
+    if (!bs) {
+        goto out;
+    }
+
+    if (blk_insert_bs(blk, bs, &err) < 0) {
+        bdrv_unref(bs);
+        goto out;
+    }
+
+    bdrv_unref(bs);
+    monitor_printf(mon, "OK\n");
+    return;
+
+out:
+    hmp_handle_error(mon, err);
+}
+
 void hmp_commit(Monitor *mon, const QDict *qdict)
 {
     const char *device = qdict_get_str(qdict, "device");
diff --git a/hmp-commands.hx b/hmp-commands.hx
index 5cc4788f12..79af8e8988 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -207,6 +207,24 @@ SRST
   actions (drive options rerror, werror).
 ERST
 
+    {
+        .name       = "drive_insert",
+        .args_type  = "id:B,filename:F",
+        .params     = "device filename",
+        .help       = "insert a host block device into an empty drive",
+        .cmd        = hmp_drive_insert,
+    },
+
+SRST
+``drive_insert`` *device* *filename*
+  Insert a host block device file into a drive that has been emptied by
+  ``drive_del``.  This reconnects the backing store without removing the
+  guest device, enabling transparent disk hot-swap for non-removable devices
+  such as NVMe namespaces.  Combined with PCIe AER Surprise Down error
+  injection (``pcie_aer_inject_error`` *device* ``SDN``), this enables
+  complete NVMe disk hot-swap simulation.
+ERST
+
     {
         .name       = "change",
         .args_type  = "device:B,force:-f,target:F,arg:s?,read-only-mode:s?",
diff --git a/include/block/block-hmp-cmds.h b/include/block/block-hmp-cmds.h
index 71113cd7ef..73c9607402 100644
--- a/include/block/block-hmp-cmds.h
+++ b/include/block/block-hmp-cmds.h
@@ -21,6 +21,7 @@ void hmp_drive_add(Monitor *mon, const QDict *qdict);
 
 void hmp_commit(Monitor *mon, const QDict *qdict);
 void hmp_drive_del(Monitor *mon, const QDict *qdict);
+void hmp_drive_insert(Monitor *mon, const QDict *qdict);
 
 void hmp_drive_mirror(Monitor *mon, const QDict *qdict);
 void hmp_drive_backup(Monitor *mon, const QDict *qdict);
-- 
2.50.1 (Apple Git-155)