/* this xor is used by the degraded-mode dag functions to recover lost
* data. the second-to-last parameter is the PDA for the failed
* portion of the access. the code here looks at this PDA and assumes
* that the xor target buffer is equal in size to the number of
* sectors in the failed PDA. It then uses the other PDAs in the
* parameter list to determine where within the target buffer the
* corresponding data should be xored. */