ixgbevf: reset unused mailbox data registers

ixgbevf: reset unused mailbox data registers

The VF_RESET message of the 82599 PF/VF communication protocol issued by a
a Guest VF driver may include an optional permanent MAC address assigned to
the VF by the Guest OS, in order to make it recorded into the 82599 RAR
registers by the Host PF driver.

To indicate the absence of this optional MAC address, the VF_RESET command
assumes that a NULL MAC address is sent, instead of using a dedicated bit
for this purpose. However, when sending a VF_RESET command with no permanent
MAC address, the function ixgbe_reset_hw_vf() of the 82599 VF driver
directly invokes the function ixgbe_write_mbx_vf() with a message that does
not include a NULL MAC address, wrongly assuming that this function fills in
with zero all unused mailbox data registers.

More globally, it is safer to explicitely reset to zero all remaining mailbox
data registers that are not used to store the content of a message, in order
to reset the data sent in a previous VF/PF exchange (in either side),
including the last exchange performed by another Guest OS to which that VF
was previously assigned.

Signed-off-by: Ivan Boule <ivan.boule@6wind.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>