An entry in the VDI block map will hold an offset to the actual block if the block is allocated, or one of two specially-interpreted values if not allocated. Using VirtualBox terminology, value VDI_IMAGE_BLOCK_FREE (0xffffffff) represents a never-allocated block (semantically arbitrary content). VDI_IMAGE_BLOCK_ZERO (0xfffffffe) represents a "discarded" block (semantically zero-filled). block/vdi knows only about VDI_IMAGE_BLOCK_FREE. Teach it about VDI_IMAGE_BLOCK_ZERO. Signed-off-by: Eric Sunshine <sunshine@sunshineco.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
				
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		Read the documentation in qemu-doc.html. Fabrice Bellard.
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