Re: [Qemu-ppc] [Qemu-devel] [PATCH v4 18/24] qdev: hotplug: provide do_unplug handler

[Igor Mammedov]

On Mon, 8 Oct 2018 14:41:50 +0200
David Hildenbrand <address@hidden> wrote:

> On 08/10/2018 14:19, Igor Mammedov wrote:
> > On Mon, 8 Oct 2018 13:47:53 +0200
> > David Hildenbrand <address@hidden> wrote:
> >   
> >>> That way using [2] and [1 - modulo it should match only concrete type]
> >>> machine would be able to override hotplug handlers for 
> >>> TYPE_VIRTIO_PMEM_PCI
> >>> and explicitly call machine + pci hotplug handlers in necessary order.
> >>>
> >>> flow would look like:
> >>>   [acpi|shcp|native pci-e eject]->  
> >>>        hotplug_ctrl = qdev_get_hotplug_handler(dev);
> >>>        hotplug_handler_unplug(hotplug_ctrl, dev, &local_err); ->
> >>>             machine_unplug()
> >>>                machine_virtio_pci_pmem_cb(): 
> >>>                   // we now that's device has 2 stage hotplug handlers,
> >>>                   // so we can arrange hotplug sequence in necessary order
> >>>                   hotplug_ctrl2 = qdev_get_bus_hotplug_handler(dev);
> >>>
> >>>                   //then do unplug in whatever order that's correct,
> >>>                   // I'd assume tear down/stop PCI device first, flushing
> >>>                   // command virtio command queues and that unplug memory 
> >>> itself.
> >>>                   hotplug_handler_unplug(hotplug_ctrl2, dev, &local_err);
> >>>                   memory_device_unplug()
> >>>     
> >>
> >> Looking into the details, this order is not possible. The unplug will
> >> essentially do a device_unparent() leading to the whole hierarchy
> >> getting destroyed. The memory_device part always has to come first.  
> > 
> > Question here is if there are anything that should be handled first on
> > virtio level before memory_device/pmem part is called?
> > If there isn't it might be fine to swap the order of unplug sequence.
> >   
> 
> Was asking myself the same thing, but as we are effectively holding the
> iothread lock and the guest triggered the unplug, I guess it is fine to
> unregister the memory region at this point.
It looks the same to me but I'm not familiar with virtio or PCI.
I'd ask Michael if it's safe thing to do.