Re: [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format specification

[Kevin Wolf]

Am 11.10.2010 19:14, schrieb Anthony Liguori:
> On 10/11/2010 11:18 AM, Anthony Liguori wrote:
>> On 10/11/2010 10:46 AM, Stefan Hajnoczi wrote:
>>> On Mon, Oct 11, 2010 at 05:39:01PM +0200, Avi Kivity wrote:
>>>>   On 10/11/2010 05:30 PM, Stefan Hajnoczi wrote:
>>>>>>   It was discussed before, but I don't think we came to a 
>>>>>> conclusion. Are
>>>>>>   there any circumstances under which you don't want to set the
>>>>>>   QED_CF_BACKING_FORMAT flag?
>>>>> I suggest the following:
>>>>>
>>>>> QED_CF_BACKING_FORMAT_RAW = 0x1
>>>>>
>>>>> When set, the backing file is a raw image and should not be probed for
>>>>> its file format.  The default (unset) means that the backing image 
>>>>> file
>>>>> format may be probed.
>>>>>
>>>>> Now the backing_fmt_{offset,size} are no longer necessary.
>>>> Should it not be an incompatible option?  If the backing disk starts
>>>> with a format magic, it will be probed by an older qemu,
>>>> incorrectly.
>>> Agreed, it should be a non-compat feature bit.
>>
>> If it's just raw or not raw, then I agree it should be non-compat.
>>
>> I think we just need a feature bit then that indicates that the 
>> backing file is non-probeable which certainly simplifies the 
>> implementation.
>>
>> QED_F_BACKING_FORMAT_NOPROBE maybe?
> 
> Er, thinking more, this is still a good idea but we still need 
> QED_CF_BACKING_FORMAT because we specifically need to know when a 
> protocol is specified.  Otherwise, we have no way of doing nbd as a 
> backing file.

Well, the protocol is currently encoded in the file name, separated by a
colon. Of course, we want to get rid of that, but we still don't know
what we want instead. It's completely unrelated to the backing file
format, though, it's about the format of the backing file name.

Kevin