[Qemu-devel] Re: RFC: QMP event notification for disk media eject

[Anthony Liguori]

On 01/11/2011 07:11 AM, Luiz Capitulino wrote:
> Hi there,
>
> I need feedback on a new QMP event.
>
> Problem
> =======
>
> There's no way for a management tool to detect that a guest OS has ejected the
> media in a CDROM or Floppy disk drive (I'm discarding polling, because it's
> undesirable at best).
>
> The end result is that the management tool can get confused, this is happening
> with libvirt when migration is involved: if the guest is saved/restored or
> migrated, then libvirt will start the guest again with media still present.
>
> NOTE: Most of the analysis here was done by Daniel Berrange.
>
> Solution
> ========
>
> We need a new QMP event to solve that. There are two possible events, a
> general one and a very specific one.
>
> There are 3 scenarios in which both events should be emitted:
>
>   1. When guest OS ejects media
>   2. When 'eject' monitor command is run
>   3. When 'change' monitor command is run
>
> BLOCK_MEDIA_CHANGE
> ------------------
>
> This is the general event, it's emitted when any removable block device
> is changed.
>
> Ideally, the event should contain two pieces of info:
>
>   - qdev device name
>   - new file path (to allow distinguishing eject from change)
>
> Example:
>
>    { "event": "BLOCK_MEDIA_CHANGE", "data": { "qdev-id": "myid",
>                                               "new-path": 
> "/foo/bar/dir/distro.iso" },
>                                               ... }
>    

I think this is short sighted as block devices are not simply expressed 
in terms of new-path.  You would need to do something like:

{ 'blockdev': {'file': '/foo/bar/dir/distro.iso', 'cache', 'off', ...}}

And that adds a lot of complexity that I'm not sure is really 
justified.  Tray status is really what you're interested in.  The user 
cannot directly change the media, only the management tools can so why 
would the management tools need to be notified about something that they 
did?

Regards,

Anthony Liguori

> BLOCK_MEDIA_EJECT
> -----------------
>
> This event is only emitted when a CDROM or a floppy disk is ejected.
>
> The event contains one info one:
>
>   - device name
>
> Example:
>
>    { "event": "BLOCK_MEDIA_EJECT", "data": { "device-name": "ide1-cd0" }, ... }
>