Re: [libvirt] [PATCH 4/4] Support for probing qed image metadata

> More worrying, I don't see anything in QED that requires the filename to > occur within the first 10K bytes. Do we need to add another enum value > to libvirt's backing store callback routine, to be used when the header > requests data that lies beyond buf_size but is still feasibly valid, for > the case where QED designates a backing store location that is beyond > 10k but still before the start of the first cluster, rather than the > current approach of just treating it as BACKING_STORE_INVALID? QED doesn't explicitly restrict backing_filename_offset/backing_filename_size to just the header cluster(s). I think it makes sense to say backing_filename_offset + backing_filename_size <= header_size * cluster_size and will add it to the QED spec.

But that doesn't guarantee backing_filename_offset < 10 KB. The space after struct QEDHeader is explicitly unstructured so extensions can put arbitrary data there in a backwards compatible way.