Re: [PATCH v0 2/2] block: allow to set 'drive' property on a realized block device

[Denis Plotnikov <dplotnikov@virtuozzo.com>]

On 16.12.2019 18:38, Kevin Wolf wrote:
> Am 16.12.2019 um 15:51 hat Denis Plotnikov geschrieben:
>> On 13.12.2019 13:32, Kevin Wolf wrote:
>>> Am 18.11.2019 um 11:50 hat Denis Plotnikov geschrieben:
>>>> Another problem here, is that the "size" of the device dev may not match
>>>> after setting a drive.
>>>> So, we should update it after the drive setting.
>>>> It was found, that it could be done by calling
>>>> BlockDevOps.bdrv_parent_cb_resize.
>>>>
>>>> But I have some concerns about doing it so. In the case of virtio scsi
>>>> disk we have the following callstack
>>>>
>>>>        bdrv_parent_cb_resize calls() ->
>>>>            scsi_device_report_change(dev, SENSE_CODE(CAPACITY_CHANGED)) ->
>>>>                virtio_scsi_change ->
>>>>                    virtio_scsi_push_event(s, dev, VIRTIO_SCSI_T_PARAM_CHANGE,
>>>>                                                        sense.asc |
>>>> (sense.ascq << 8));
>>> I think the safest option for now (and which should solve the case you
>>> want to address) is checking whether old and new size match and
>>> returning an error otherwise.
>>>
>>>> virtio_scsi_change  pushes the event to the guest to make the guest
>>>> ask for size refreshing.  If I'm not mistaken, here we can get a race
>>>> condition when some another request is processed with an unchanged
>>>> size and then the size changing request is processed.
>>> I think this is actually a problem even without resizing: We need to
>>> quiesce the device between removing the old root and inserting the new
>>> one. They way to achieve this is probably by splitting blk_drain() into
>>> a blk_drain_begin()/end() and then draining the BlockBackend here while
>>> we're working on it.
>>>
>>> Kevin
>> Why don't we use bdrv_drained_begin/end directly? This is what
>> blk_drain does.
>> If we want to split blk_drain we must keep track if blk's brdv isn't
>> change otherwise we can end up with drain_begin one and drain end
>> another bdrv if we do remove/insert in between.
> Hmm, true, we would have to keep track of draining at the BlockBackend
> level and consider it in blk_remove_bs() and blk_insert_bs(). Maybe
> that's not worth it.
>
> If we use bdrv_drained_begin/end directly, I think we need to drain both
> the old and the new root node during the process.
>
>> Another thing is should we really care about this if we have VM
>> stopped and the sizes matched?
> How do we know that the VM is stopped? And why would we require this?
I implied the scenario of VM migration over a shared storage with an 
exclusive file access model.
The VM is stopped on drive changing phase.

If there is no use to require it, than ok.

Denis
> Your patch doesn't implement or at least check this, and it seems a bit
> impractical for example when all you want is inserting a filter node.
>
> Kevin