Re: [Qemu-devel] [Nbd] [PATCH 2/2] NBD proto: add GET_LBA_STATUS extension

[Paolo Bonzini]

On 24/03/2016 11:32, Alex Bligh wrote:
>> > Now I'm not saying we
>> > need to fully define what it means for a part of the backend to be
>> > "dirty" or not.  It's okay to leave part of the meaning in the dark,
>> > leaving that implementation-defined.
> Well, the 3 possible states are:
> 
> * unallocated
> * zero
> * non-zero
> 
> So the possible replies are a bitfield of those, with a '1' if it 'might'
> be in that state, i.e.
> 
> 111 = no idea
> 110 = might be zero or unallocated, but isn't zero
> 011 = I know it's allocated, but I don't know whether it is zero or not

How do you represent "definitely unallocated?"

Paolo

> And '000' is not permitted!