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

[Luiz Capitulino]

On Fri, 27 Jan 2012 10:52:15 +0100
Kevin Wolf <address@hidden> wrote:

> Am 26.01.2012 18:57, schrieb Luiz Capitulino:
> > On Wed, 25 Jan 2012 10:42:04 -0200
> > Luiz Capitulino <address@hidden> wrote:
> > 
> >> On Wed, 25 Jan 2012 09:41:20 +0100
> >> Kevin Wolf <address@hidden> wrote:
> >>
> >>> Am 24.01.2012 21:03, schrieb Eric Blake:
> >>>> On 01/24/2012 11:16 AM, Luiz Capitulino wrote:
> >>>>> Libvirt wants to be notified when the guest ejects a medium, so that
> >>>>> it can update its view of the guest.
> >>>>>
> >>>>> This code has been originally written by Daniel Berrange. It adds
> >>>>> the event to IDE and SCSI emulation.
> >>>>>
> >>>>> 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.
> >> The exception is if it allows HMP to run in parallel with QMP, but I don't 
> >> think
> >> this is recommended (at least not for commands that change any VM state).
> > 
> > Let me elaborate more. We have two options:
> > 
> >  1. Emit the event for guest initiated ejects (this patch, although I think
> >     the event should be renamed to GUEST_MEDIUM_EJECT)
> > 
> >  2. Emit the event for guest & QMP initiated ejects, that's, also emit the
> >     event for the eject and change commands
> > 
> > The first thing to note is that, they're not mutually exclusive. If we do
> > item 1 now, we can add 2 later (as a different event).
> > 
> > But my point is that doing 2 is problematic and we should avoid it. The 
> > problem
> > is that the semantics of eject and change are complex and/or buggy. And 
> > something
> > I've learned about confusing semantics in QMP is that, they will give you 
> > headaches
> > soon or later.
> 
> But I'm not really interested in the semantics of QMP commands, because
> they are irrelevant for the events.

I do think they are relevant, because the event will have to match what
the eject/change commands do with the tray. If what they do is messy, the
event will turn out to be messy too.

Now, I don't doubt this can be all fixed and made clean. I'm just not sure
if it's worth it.

> My view is that a device generates an event each time its try status
> changes.

Agreed.

> If it opens the tray, it does so by calling bdrv_eject - an
> obvious place to send an event. 

Yes, bdrv_eject() is called from device models. That's the easy part. But note
that device models will also call bdrv_eject() when closing the tray.

> If it closes the tray or the eject
> monitor command is used, we go through bdrv_dev_change_media_cb - not
> quite as obvious, but I think this despite its name this is really a
> "tray status changed externally" callback.

Not so simple for the eject command, because bdrv_dev_change_media_cb() is
called from bdrv_close() but if the tray is locked by the guest and it later
unlocks it (and that _will_ happen with you run eject with the tray locked
by the guest) the event will be also emitted.

> QMP commands may cause any of these actions to occur. But the event is
> tied to the actions and not to the QMP commands that may or may not
> cause them according to their confusing semantics.
> 
> > The main problem with eject is that it's inconsistent with the handling of
> > the tray status. Try this:
> > 
> > 1. Eject with no medium inserted
> > 
> > (qemu) info block
> > ide1-cd0: removable=1 locked=0 tray-open=0 io-status=ok [not inserted]
> > (qemu) eject ide1-cd0
> > (qemu) info block
> > ide1-cd0: removable=1 locked=0 tray-open=0 io-status=ok [not inserted]
> > (qemu) 
> > 
> > Conclusion: the tray didn't move, we shouldn't emit the event.
> 
> Fails before bdrv_close, so bdrv_dev_change_media_cb is never called.
> Works as it is today.

Yes, my point with these two examples is that, ideally, eject should left
the tray in the same state when it's issued. Today the tray status after
eject depends on if a media is inserted or not.

Maybe not a big issue, but I felt I should raise it.

> 
> > 
> > 2. Eject with medium inserted
> > 
> > (qemu) info block
> > ide1-cd0: removable=1 locked=0 tray-open=0 io-status=ok file=/tmp/vl.iKvBAF 
> > backing_file=/mnt/fernando/isos/linux/Fedora-14-x86_64-DVD.iso ro=0 
> > drv=qcow2 encrypted=0 bps=0 bps_rd=0 bps_wr=0 iops=0 iops_rd=0 iops_wr=0
> > (qemu) eject ide1-cd0
> > (qemu) info block
> > ide1-cd0: removable=1 locked=0 tray-open=1 io-status=ok [not inserted]
> > (qemu) 
> > 
> > Conclusion: the tray opened and the media was purged. We should emit the 
> > event.
> 
> bdrv_dev_change_media_cb is called, we can emit an event there.
> 
> > 
> > 3. Eject with medium inserted and locked
> > 
> > (qemu) info block
> > ide1-cd0: removable=1 locked=1 tray-open=0 io-status=ok file=/tmp/vl.2LHApn 
> > backing_file=/mnt/fernando/isos/linux/Fedora-16-x86_64-DVD.iso ro=0 
> > drv=qcow2 encrypted=0 bps=0 bps_rd=0 bps_wr=0 iops=0 iops_rd=0 iops_wr=0
> > (qemu) eject ide1-cd0
> > Device 'ide1-cd0' is locked
> > (qemu) info block
> > ide1-cd0: removable=1 locked=0 tray-open=1 io-status=ok file=/tmp/vl.2LHApn 
> > backing_file=/mnt/fernando/isos/linux/Fedora-16-x86_64-DVD.iso ro=0 
> > drv=qcow2 encrypted=0 bps=0 bps_rd=0 bps_wr=0 iops=0 iops_rd=0 iops_wr=0
> > (qemu) 
> > 
> > Conclusion: eject returned an error, but a few seconds later the tray 
> > opened and
> >             the media wasn't purged. What happened here is that, the _guest_
> >             opened the tray. The code in this patch would trigger the 
> > event, but
> >             we shouldn't emit it twice if we cover eject & change (ie. 
> > special case)
> 
> bdrv_dev_change_media_cb is not called because media cannot be ejected
> with a locked drive. Instead bdrv_dev_eject_request is called which
> doesn't emit an event.
> 
> If the guest happens to initiate an eject itself after receiving the
> eject request, it calls bdrv_eject, where we can emit an event.
> 
> If we had force=true in the initial eject command, bdrv_close is called,
> which in turn goes through bdrv_dev_change_media_cb where an event is
> emitted.

Can't this race with the guest eject?

> 
> > 
> > Each of the three cases has different semantics wrt to the tray. We could 
> > try
> > to fix them, but this seems to require some surgery... Is it worth it?
> > 
> > Now, the change command. Basically, it opens the tray, purges the medium,
> > inserts another medium and closes the tray. Should we generate the event 
> > twice?
> 
> change is eject followed by bdrv_open. We don't teleport media into a
> device with closed tray, so yes, this is an open event followed by a
> close event.
> 
> > So, I'd just avoid all this and have only GUEST_MEDIUM_EJECT, which has 
> > clear
> > semantics and satisfies libvirt needs...
> 
> I think it's rather complex semantics compared to doing it always: Two
> more or less obvious places in block.c and no modification of the device
> emulation needed. For me that's a clear winner.
> 
> Kevin
>