Re: [Qemu-devel] [PATCH RFC] block: add block-insert-node QMP command

[Manos Pitsidianakis <el13635@mail.ntua.gr>]

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On Wed, Aug 16, 2017 at 06:59:25AM -0500, Eric Blake wrote:
>On 08/16/2017 04:41 AM, Manos Pitsidianakis wrote:
>
>>>> +##
>>>> +# @block-insert-node:
>>>> +#
>>>> +# Insert a filter node between a specific edge in the block driver
>>>> state graph.
>>>> +# @parent:  the name of the parent node or device
>>>> +# @node:    the name of the node to insert under parent
>>>> +# @child:   the name of the child of both node and parent
>>>
>>> Is this always going to be between two existing nodes, or can this
>>> command also be used to insert at the end of the chain (for example, if
>>> parent or child is omitted)?
>>
>> If this is used for filter nodes, I suppose only between would make
>> sense (for now). Is there a use case for the latter?
>
>Perhaps.
>
>Given a qcow2 image backing chain:
>
>base <- active
>
>there are four BDS (2 format, 2 protocol).  Ideally, I could add
>filtering to any one of those four nodes (a filter on the base protocol
>level restricts how much guest data can be used from the backing image,
>but with no limits on the qcow2 metadata; a filter on the base format
>level restricts metadata reads as well; similarly for filters on the
>active protocol and format layers).
>
>But adding a filter on 'active' at the format level has no pre-existing
>parent (I'm adding the filter as the new top-level).  Or am I missing
>something?

The parent in this case is the storage device (disk / cdrom), whose name 
is specified as the parent. The first example in the 
qapi/block-core.json is such a case. In code I check blk_by_name(parent) 
and if that doesn't exist, I try with bdrv_find_node(parent).  Perhaps I 
should reword the documentation or did I misunderstand what you wrote?

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