On Apr 28 15:00, Max Reitz wrote:
>On 28.04.21 12:12, Klaus Jensen wrote:
>>On Apr 28 09:31, Oguz Bektas wrote:
>>>>My understanding is that this is the expected behavior. The reason is
>>>>that the drive cannot be deleted immediately when the device is
>>>>hot-unplugged, since it might not be safe (other parts of QEMU could
>>>>be using it, like background block jobs).
>>>>
>>>>On the other hand, the fact that if the drive is removed explicitly
>>>>through QMP (or in the monitor with drive_del), the drive id is
>>>>remains "in use". This might be a completely different bug that is
>>>>unrelated to the nvme device.
>>>
>>>using the same commands I can hot-plug and hot-unplug a scsi disk like
>>>this without issue - this behavior only appeared on nvme devices.
>>>
>>
>>Kevin, Max, can you shed any light on this?
>>
>>Specifically what the expected behavior is wrt. to the drive when 
>>unplugging a device that has one attached?
>>
>>If the scsi disk is capable of "cleaning up" immediately, then I 
>>suppose that some steps are missing in the nvme unrealization.
>

Hi Max,

Thanks for your help!

>I’m not very strong when it comes to question for guest devices, but 
>looking into the QAPI documentation, it says that when DEVICE_DELETED 
>is emitted, it’s safe to reuse the device ID.  Before then, it isn’t, 
>because the guest may yet need to release the device.
>

This is specifically related to releasing the "drive", not the device. 
Problem is that when the device is deleted (using device_del), the pci 
device goes away rapidly, but the drive (as shown in `info block`) 
lingers for a couple of seconds before going into the "unlocked" state. 

If the drive is then deleted (`drive_del`) everything is good, but if 
the drive is deleted within those couple of seconds, the drive_del 
completes successfully, but the drive id never becomes available again.

>So the questions that come to my mind are:
>- Do nvme guest devices have a release protocol with the guest, so 
>that it just may take some time for the guest to release the device?  
>I.e. that this would be perfectly normal and documented behavior?
>(Perhaps this just isn’t the case for scsi, or the guest just releases 
>those devices much quicker)
>

The NVMe device is a PCIDevice, I wonder if that is what adds some delay 
on this?

>- Did Oguz always wait for the DEVICE_DELETED event before reusing the 
>ID?  Sounds like it would be a bug if reusing the ID after getting 
>that event failed.
>

 From the bug report, it does not look like anything like that is done.

I basically dont understand the deletion protocol here and why the drive 
is not released immediately. Even if I add a call to 
blockdev_mark_auto_del() there is a delay, but the drive does get 
automatically deleted.