Osier Yang<address@hidden> 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.