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Date: Mon, 11 Oct 2010 12:14:45 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] Re: [PATCH v2 3/7] docs: Add QED image format
	specification
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To: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Christoph Hellwig <hch@lst.de>, Avi Kivity <avi@redhat.com>, qemu-devel@nongnu.org

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.

Regards,

Anthony Liguori

> Regards,
>
> Anthony Liguori
>
>> Stefan
>>
>