Re: [Qemu-block] [Qemu-devel] [PATCH v10 09/16] block: Add QMP support for streaming to an intermediate layer

[Alberto Garcia]

On Mon 10 Oct 2016 09:09:25 PM CEST, Eric Blake wrote:
>>  # @job-id: #optional identifier for the newly-created block job. If
>>  #          omitted, the device name will be used. (Since 2.7)
>>  #
>> -# @device: the device name or node-name of a root node
>> +# @device: the device or node name of the top image
>>  #
>>  # @base:   #optional the common backing file name
>>  #
>> -# @backing-file: #optional The backing file string to write into the active
>> -#                          layer. This filename is not validated.
>> +# @backing-file: #optional The backing file string to write into the top
>> +#                          image. This filename is not validated.
>
> No change to the actual introspection. What's the easiest way for
> libvirt to learn if the new style command will work, short of actually
> trying it to see if it succeeds or fails?  (That may be sufficient,
> but the error message quality can often be improved in libvirt if it
> is known up front if qemu is new enough rather than taking the 'try
> and see' approach and getting stuck with qemu's error message)

I don't think there's any straightforward way. Some ideas:

1) Try to start a block-stream job that does nothing. When base ==
backing_bs(device) that's a no-op, but it fails if device is not the
root node and intermediate block streaming is not supported.

2) We could add an extra parameter. For example 'base-node', which would
be the same as 'base' but would take a node name instead of a file name.

3) QEMU could advertise that feature to the client. This is probably
simpler than trying to figure it out from the API. I guess that's the
idea of 'qmp_capabilities'?

Berto