Re: Question about scsi device hotplug (e.g scsi-hd)

[Stefan Hajnoczi]

On Tue, Mar 31, 2020 at 07:16:23PM +0300, Maxim Levitsky wrote:
> Hi!
> 
> I recently investigated an interesting issue related to repeated scsi-hd 
> hotplug/hotunplug.
> The bugzilla for it is here:
> https://bugzilla.redhat.com/show_bug.cgi?id=1812399
> 
> In nutshell the issue that I think that I found and I would like to ask your 
> opinion on it,
> since I don't have enough experience to be 100% sure that I didn't miss 
> something  is this:
> 
> When a new device is hotplugged via monitor, the qdev_device_add first puts 
> the device on
> the bus where the user requested it to be, and then calls the device's 
> .realize.
> 
> However for scsi bus, each time a new request is sent from the guest, the 
> scsi adapter drivers
> (e.g virtio-scsi) call scsi_device_find to find the LUN's driver to dispatch 
> the request to,
> and scsi_device_find will return the added device as soon as it is placed on 
> the bus.
> 
> Thus between the point when the new device is placed on the bus and until the 
> end of the .realize,
> the device can be accessed by the guest when it is not yet realized or 
> partially realized as
> happens in the bugreport.
> 
> What do you think about it?

Maybe aio_disable_external() is needed to postpone device emulation
until after realize has finished?

Virtqueue kick ioeventfds are marked "external" and won't be processed
while external events are disabled.  See also
virtio_queue_aio_set_host_notifier_handler() ->
aio_set_event_notifier().

Stefan

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