Re: [Qemu-devel] [PATCH] qmp: add BLOCK_MEDIUM_EJECT event

[Paolo Bonzini]

On 01/25/2012 02:23 PM, Kevin Wolf wrote:
> > > >  Please, note that this only covers guest initiated ejects, that's,
> > > >  the QMP/HMP commands 'eject' and 'change' are not covered.
> > >
> > >  What's the reason for this behaviour? It feels inconsistent.
> >
> >  I don't think it's inconsistent because we're limiting it to guest initiated
> >  actions. Also, the mngt app knows when it sends a 'eject' or 'change' 
> command.
>
> Yes, management shouldn't need an event in order to see what it just did
> itself. I haven't really looked at the code yet, but what I want to
> avoid is that we have to pass a flag all the way through qemu just so we
> don't send an event in the end. This might not be the case today, but
> after some more cleanup of the code it might just turn out like this.

Management must be ready anyway to receive an event in response to 
eject/change actions that it started, because the guest might be 
trapping the host's eject requests ("press the button") and turning them 
into guest-initiated ejects.  This is what recent udev does.

Thus, a reliable eject procedure must be as follows:

1) Eject the disc

2) query the state of the tray.  If it is open, poll for eject events 
and consume them.  If it is closed, either exit or wait for an eject 
event to arrive.

We can document that the event MAY NOT be reported for host-initiated 
ejects.  It is future-proof and actually closer to what happens in 
practice if you consider a wide range of guests.

Paolo