Re: [Qemu-block] [PATCH RFC 1/1] Stream block job involves copy-on-read filter

[Alberto Garcia]

On Mon 11 Feb 2019 03:51:33 PM CET, Vladimir Sementsov-Ogievskiy wrote:
>>>> I think this was already discussed in the previous version of this
>>>> patch: if you insert a copy-on-read filter here then all guest
>>>> reads will copy the data from the backing chain, but you don't want
>>>> to copy anything below the 'base' node, so the copy-on-read filter
>>>> needs a 'base' parameter.
>>>>
>>> Before that, we still have the unresolved issue with getting the
>>> filter as the 'base' input parameter. That happens sometimes because
>>> the base is being searched by the file name, which is the same to
>>> the one of the filter, rather than by the node name (!)
>> 
>> I'm not sure if I understand. block-stream has 'base' and 'base-node'
>> parameters, and in the first case you would convert the file name to
>> a node name.
>> 
>> stream_start() gets the BlockDriverState, not the file name, so why
>> can't you get the node name from there and pass it to the
>> copy-on-read filter?
>
> The problem is in the concept of "base" node. The code written in
> manner that base is not changed during block job. However, job don't
> own base and there is no guarantee that it will not change during the
> job.

But if that's the case then we have a problem already, because 'base' is
a member of StreamBlockJob and is used in the existing stream_run()
code.

So if there's a way to make 'base' disappear during the job (how?) then
we could protect it with block_job_add_bdrv().

Berto