Re: Patching guest kernel code for better performance from HOST

[Dushyant Bansal <cs5070214@cse.iitd.ac.in>]

Hi,

On patching 'mfmsr' instruction with 'lwz', guest exits when it tries to 
execute that 'lwz' instruction. I am looking for possible causes for 
this exit.

Here are the details:
Initially,
pc: 0xc0019420, instruction: 0x7c0000a6 [mfmsr r0]
As this is a privileged instruction, this causes an exit.

qemu-system-ppc-4443  [000] 19733.740013: kvm_book3s_exit: exit=0x700 | 
pc=0xc0019420 | inst=0x7c0000a6 | msr=0x1032 | dar=0xe1736a00 | 
srr1=0x100000000004d032
  qemu-system-ppc-4443  [000] 19733.740029: kvm_book3s_patch: return=0 | 
pc=0xc0019420 | inst=0x7c0000a6 | msr=0x1032 | new_inst=0x8000f05c
  qemu-system-ppc-4443  [000] 19733.740030: kvm_ppc_instr: inst 
2080374950 pc 0xc0019420 emulate 0
  qemu-system-ppc-4443  [000] 19733.740037: kvm_book3s_reenter: reentry 
r=1 | pc=0xc0019420

I patched this instruction with:
0x8000f05c:     lwz    r0, -4096(offset of msr)
This instruction reads the 'msr' field of the magic page into register r0.

Then, I do not increment the pc value, so the guest starts at the same 
pc which now points to the new patched instruction.

This 'lwz' instruction is causing a exit due to 
'BOOK3S_INTERRUPT_PROGRAM' (exit_nr: 0x700).
What could be the reason for this exit? As, 'lwz' is not a privileged 
instruction, I am unable to think of any reason.

  qemu-system-ppc-4443  [000] 19733.740040: kvm_book3s_exit: exit=0x700 
| pc=0xc0019420 | inst=0x8000f05c | msr=0x1032 | dar=0xf00000000105d720 
| srr1=0x100000000004d032
  qemu-system-ppc-4443  [000] 19733.740042: kvm_ppc_instr: inst 
2147545180 pc 0xc0019420 emulate 1
  qemu-system-ppc-4443  [000] 19733.740043: kvm_book3s_reenter: reentry 
r=3 | pc=0xc0019424
  qemu-system-ppc-4443  [000] 19733.740046: kvm_userspace_exit: reason 
KVM_EXIT_MMIO (6)


Any help/clue would be greatly appreciated.

Thanks,
Dushyant