After a global reset to the core, the lower bits of the statistics counters are reset.
Per the IEEE 802.3 specification, sub-clause 5.2.1, the statistic counters are non-resettable wraparound counters. This means that they are not reset after a READ operation, and they will only return to zero when they naturally wrap around or when the device is reconfigured.
This applies to v8.6 and earlier of the 10-Gigabit Ethernet MAC. For counters counting the number of frames for a particular statistic, the lower three bits (2:0) are cleared. For example, register 0x000, 001, 003, 0004, 005, 020, 025 and others.
For counters counting the number of bytes for a particular statistic, the lower 9 bits (8:0) are cleared. For example, register 0x012, 013 and others.
This issue can be worked around by using either of the following methods:
1) The problem with the statistics counter bits being cleared does NOT occur if you just individually reset the RX and TX via bit 31 of register 0x240 and 0x280. The individual RX and TX resets could be used instead of the main reset to the core.
2) Additional user logic could be added to read the statistics before doing a reset to track the value that gets cleared from the statistics register.