We are porting gem5 to support MIPS64, and find some problems. We try to fix them, as shown below:

1. fetch_seq_unit.cc
Comment 2 lines. pc shouldn't be changed at this condition;
2. branch_predictor.cc
Comment 1 line. By default, set target to NPC is OK.
Adding a condition for MIPS branch likely instruction.
3. bpred_unit.cc
If the branch is not taken, BTB should not be updated.

In addition, 2 places in src/arch/mips/isa/formats/branch.isa:
elif x == 'Likely':
   # not_taken_code = 'NNPC = NPC, NPC = PC;'
The not_taken_code should be commented.

We have run many benchmarks of MIPS64 in gem5.
And for MIPS32, with CPU_MODELS=InOrderCPU, hello world run correctly.