Re: [Qemu-devel] [PATCH v2 10/11] block: add option 'backing' to -drive options

[Paolo Bonzini <pbonzini@redhat.com>]

Il 17/07/2013 14:58, Kevin Wolf ha scritto:
> Am 17.07.2013 um 14:36 hat Paolo Bonzini geschrieben:
>> Il 17/07/2013 11:42, Fam Zheng ha scritto:
>>> This option allows overriding backing hd of drive. If the target drive
>>> exists, it's referenced as the backing file and refcount incremented.
>>>
>>> Example:
>>>     qemu-system-x86_64 -drive \
>>>         file.filename=foo.qcow2,if=none,id=foo \
>>>         -drive file=bar.qcow2,backing=foo
>>
>> I guess this is where we need the soft reference.
>>
>> This has a _lot_ of potential for misuse, I think Kevin bashed me and
>> Federico very heavily when we tried to do something similar.
> 
> Not sure what exactly I "bashed" you for

Doing strange things with bs->backing_hd (blkmirror comes to mind).

> This is basically restarting the discussion where I suggested to give
> the targets of a block job names so that they can be reused. It's about
> the same kind of misuse that becomes possible and that we need to
> protect against.

Yes.  But then I'm not sure why we need to rush in blockdev-backup now.
 Instead we can simply make drive-backup optionally give a name to the
target.

I understand this is the right thing to do long term, but pre-opening of
the target is not really needed for fleecing.

Paolo