1. A method for data transfer to memory over an input/output (I/O) interconnect, comprising: reading a mailbox stored on an input/output (I/O) device in response to a request to initiate an input/output (I/O) transaction, the I/O device external to all hub devices and the mailbox storing a directive that defines a condition under which cache injection for data values in the I/O transaction will not be performed; embedding a hint into the I/O transaction when the directive in the mailbox matches data received in the request; and executing the I/O transaction; wherein execution of the I/O transaction causes a system chipset or an input/output (I/O) hub, for a processor receiving the I/O transaction, to directly store the data values from the I/O transaction into system memory and to suppress the cache injection of the data values into a cache memory, the suppression of the cache injection occurring responsive to detecting the hint in a header of the I/O transaction and without disabling the cache memory.