Re: [Qemu-devel] [PATCH v2] RFC: Add blockdev-del QMP command

[Kevin Wolf]

Am 12.02.2014 um 18:36 hat Ian Main geschrieben:
> This is the sister command to blockdev-add.  In Fam's example he uses
> the drive_del HMP command to clean up but it would be much nicer to
> have a way to do this via QMP.
> 
> Signed-off-by: Ian Main <address@hidden>

> diff --git a/qapi-schema.json b/qapi-schema.json
> index d22651c..01186cd 100644
> --- a/qapi-schema.json
> +++ b/qapi-schema.json
> @@ -4469,3 +4469,14 @@
>  # Since: 1.7
>  ##
>  { 'command': 'blockdev-add', 'data': { 'options': 'BlockdevOptions' } }
> +
> +##
> +# @blockdev-del:
> +#
> +# Delete a block device.
> +#
> +# @device: Identifier for the block device to be deleted.
> +#
> +# Since: 2.0
> +##
> +{ 'command': 'blockdev-del', 'data': { 'device': 'str' } }

I believe the full documentation should go here as well, not just in
qmp-commands.hx.

> diff --git a/qmp-commands.hx b/qmp-commands.hx
> index c3ee46a..f08045d 100644
> --- a/qmp-commands.hx
> +++ b/qmp-commands.hx
> @@ -3442,6 +3442,36 @@ Example (2):
>  EQMP
>  
>      {
> +        .name       = "blockdev-del",
> +        .args_type  = "device:s",
> +        .mhandler.cmd_new = qmp_marshal_input_blockdev_del,
> +    },
> +
> +SQMP
> +blockdev-del
> +------------
> +
> +Remove host block device.  The result is that guest generated IO is no
> +longer submitted against the host device underlying the disk.  Once a
> +drive has been deleted, the QEMU Block layer returns -EIO which results
> +in IO errors in the guest for applications that are reading/writing to
> +the device.  These errors are always reported to the guest, regardless
> +of the drive's error actions (drive options rerror, werror).

I think we wanted to have different semantics for blockdev-del.
Specifically, it is a backend command that should have no effect on
users of that backend.

Let me paste and comment on some notes I made in a previous blockdev
discussion:

    * Make sure that an explicit blockdev-del is needed to remove the
      BDS; it shouldn't happen automagically just because the guest
      device was unplugged
      [done]

    * By default, return an error for blockdev-del if reference count > 1
      [ The assumption is here that one reference is held by the
        monitor/external user, which isn't true today to my knowledge ]

      - But have a force option that closes the image file, even if it
        breaks the remaining users (e.g. uncooperative guest that doesn't
        release its PCI device)
        [ Here we need working refcounting including the external user,
          because otherwise we don't free the (closed) BDS even when the
          guest device is unplugged. It is an open question whether and
          how BDSes without an external reference are shown in the
          monitor. ]

    * Prevent mixing blockdev-add with drive_del and vice versa

      - Ideally drive_add BDSes are exactly those with a DriveInfo
        [ not true today, but DriveInfo.enable_auto_del can be used to
          distinguish them ]

So I believe we still have some design work to do before we can actually
implement this. I would prefer not to merge this for 2.0.

Kevin