virtio_net: Add a virtqueue for outbound control commands
This will be used for RX mode, MAC filter table, VLAN filtering, etc... The control transaction consists of one or more "out" sg entries and one or more "in" sg entries. The first out entry contains a header defining the class and command. Additional out entries may provide data for the command. The last in entry provides a status response back from the command. Virtqueues typically run asynchronous, running a callback function when there's data in the channel. We can't readily make use of this in the command paths where we need to use this. Instead, we kick the virtqueue and spin. The kick causes an I/O write, triggering an immediate trap into the hypervisor. Signed-off-by: NAlex Williamson <alex.williamson@hp.com> Acked-by: NRusty Russell <rusty@rustcorp.com.au> Signed-off-by: NDavid S. Miller <davem@davemloft.net>
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