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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Apr 09, 2026 at 08:01:54AM +0200, mr-083 wrote:
> 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 +++++++++++

I see v3 has dropped this new command, but in case you have plans
to re-introduce it..

First no new HMP-only commands please. Anything new must be implemented
in QMP, and the HMP must be a shim to the QMP.

drive_insert semantics are somewhat overloaded - it is doing two jobs,
first creating the block backend, and then associating the backend with
a device.  IMHO those tasks should be separate.  "drive_add" or
"blockdev-add" can already do the first task, so we shouldn't replicate
that. AFAICT, it would only need a way to do the backend/frontend
association, and possibly the PCI error injection should be done as
a standard part of that re-association of backends, which would avoid
the need for pcie_aer_inject_error which has no QMP impl currently.



>  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)
> 
> 

With regards,
Daniel
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