Re: Patching guest kernel code for better performance from HOST

[Alexander Graf <agraf@suse.de>]

On 07.05.2011, at 22:32, Dushyant Bansal wrote:

> 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.

Did you flush the icache after you patched the instruction? See the function flush_icache_range. Without, your CPU still has the old instruction in its cache, making it trap again :).


Alex