Re: [Qemu-devel] [PATCH V6 8/8] block: Use graph node name as reference in bdrv_file_open().

[Max Reitz <mreitz@redhat.com>]

On 28.01.2014 01:04, Benoît Canet wrote:
> Le Monday 27 Jan 2014 à 20:11:59 (+0100), Max Reitz a écrit :
>> On 27.01.2014 15:36, Benoît Canet wrote:
>>> Le Friday 24 Jan 2014 à 15:54:39 (+0100), Max Reitz a écrit :
>>>> On 24.01.2014 15:48, Kevin Wolf wrote:
>>>>> Am 24.01.2014 um 14:37 hat Max Reitz geschrieben:
>>>>>> On 24.01.2014 14:26, Kevin Wolf wrote:
>>>>>>> Am 23.01.2014 um 21:31 hat Benoît Canet geschrieben:
>>>>>>>> Signed-off-by: Benoit Canet <benoit@irqsave.net>
>>>>>>>> ---
>>>>>>>>   block.c | 6 +++---
>>>>>>>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>>>>>> I'm not going to merge this one yet. It breaks qemu-iotests case 071,
>>>>>>> which would have to be adapted.
>>>>>>>
>>>>>>> However, first of all I'd like to hear the opinions of at least Eric and
>>>>>>> Max on what BlockRef should really refer to. I think node names make
>>>>>>> most sense, but perhaps it's a bit inconvenient and the command line
>>>>>>> should default to node-name = id when id is set, but node-name isn't?
>>>>>> The QAPI schema is pretty clear about this: “references the ID of an
>>>>>> existing block device.”
>>>>> Sure, that's because I wrote that text before we had a node name.
>>>>>
>>>>> However, in 1.7 references didn't work yet, so we still have all freedom
>>>>> to change the interface as we like.
>>>> Yes, that's right.
>>>>
>>>>>> However, if the ID cannot be found, I think
>>>>>> we should interpret it as a reference to the node name.
>>>>>>
>>>>>> Therefore, I'd first try bdrv_find() and if that returns NULL, try
>>>>>> again with bdrv_find_node().
>>>>> I think I would prefer to avoid such ambiguities. Otherwise a management
>>>>> tool that wants to use the node name needs to check first if it's not
>>>>> already used as a device name somewhere else and would therefore operate
>>>>> on the wrong device.
>>>>>
>>>>> On the other hand, a management tool using the same names for devices
>>>>> and nodes just gets what it deserves.
>>>>>
>>>>> Perhaps we should use a common namespace for both, i.e. you get an error
>>>>> if you try to assign a node name that is already a device name and vice
>>>>> versa?
>>>> This is what I would go for. However, then I don't really know why
>>>> we should separate the ID and the node name in the first place
>>>> (although that's probably because I haven't followed the discussion
>>>> around node names).
>>>>
>>>> Max
>>> Ping,
>>>
>>> I still want to make quorum merge.
>>> What should be done for the references ?
>>>
>>> Best regards
>>>
>>> Benoît
>> My only problem is that I don't really know what IDs are for, then. ;-)
>>
>  From the understanding I have ID are for block backend top level bds and
> node-name naming all the bds burried in the graph.
>
> So my personal opinion would be to relax the constraint on bdrv_lookup_bs
> and use it for references.
>
> Kevin && Max: what do you think of this scheme ?

I agree. For example, we could change the constraint to report an error 
only if both ID and node name are actually valid (and point to different 
devices), that is, bdrv_find() and bdrv_find_node() return different 
non-NULL values.

Max