[libvirt] [Qemu-devel] [PATCH] qdev: DEVICE_DELETED event

[Markus Armbruster]

Osier Yang <jyang at redhat.com> writes:

> I'm wondering if it could be long time to wait for the device_del
> completes (AFAIK from previous bugs, it can be, though it should be
> fine for most of the cases). If it's too long, it will be a problem
> for management, because it looks like hanging. We can have a timeout
> for the device_del in libvirt, but the problem is the device_del
> can be still in progress by qemu, which could cause the inconsistency.
> Unless qemu has some command to cancel the device_del.

I'm afraid cancelling isn't possible, at least not for PCI.

Here's how device_del works for PCI when it works, roughly:

1. device_del asks the device model to unplug itself.

2. PCI device models delegate the job to the device model providing
   their PCI bus.  Let's assume it's our PIIX3/PIIX4 mongrel.  That one
   puts an unplug request into PIIX4 function 3 where guest ACPI can see
   it, and triggers its interrupt.  Then it immediately sends the QMP
   success reply.

3. Guest ACPI (SeaBIOS) services the interrupt.  It finds the unplug
   request, and asks the guest OS nicely to give up the device.

4. If the guest OS has a working ACPI driver, and it feels like giving
   up the device, it does so, and tells ACPI when it's done.

5. Guest ACPI cleans up whatever it needs cleaned up, and signals
   successful unplug by writing the slog number to a PIIX4 function 3
   register.

6. The PIIX device destroys the device in that slot.

I call this the ACPI unplug dance.

We don't control steps 3..5.

There's no way for the guest to tell us "I got your unplug request, but
I'm not going to honor it".  Even if their was, a guest without a
working ACPI driver wouldn't use it, so we couldn't rely on it anyway.

There's no way for us to tell the guest "I changed my mind on this
unplug".  All we can do is wait and see.  Either the device goes away,
or it stays.

Native PCIe is different, I'm told, but I know even less of that than I
know of PCI/ACPI.