/* 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. */