Abstract: An I/O adapter sharing mechanism in a logically partitioned computer system allows sharing a network I/O adapter between logical partitions in an efficient manner. A first logical partition owns (or controls) the I/O adapter, and a second logical partition desires to use (or share) the I/O adapter. An I/O adapter device driver that includes a hardware interface is provided in the first logical partition. A virtual device driver is provided in the second logical partition that provides a set of functions that is at least partially defined by querying the I/O adapter device driver in the first logical partition. The I/O adapter sharing mechanism includes a transfer mechanism that allows data to be transferred directly from the virtual device driver in the second logical partition to the I/O adapter without passing through the I/O adapter device driver in the first logical partition.