Re: Patching guest kernel code for better performance from HOST

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

>>>> --- a/arch/powerpc/kvm/book3s_emulate.c
>>>> +++ b/arch/powerpc/kvm/book3s_emulate.c
>>>> @@ -88,6 +88,23 @@ int kvmppc_core_emulate_op(struct kvm_run *run, 
>>>> struct kvm_vcpu *vcpu,
>>>>                 case OP_31_XOP_MFMSR:
>>>>                         kvmppc_set_gpr(vcpu, get_rt(inst),
>>>>                                        vcpu->arch.shared->msr);
>>>> +
>>>> +                       pc = kvmppc_get_pc(vcpu);
>>>> +
>>>> +                       //if pte.may_write=false then, kvmppc_st 
>>>> will not work
>>>> +                       kvmppc_xlate(vcpu,pc,false,&pte);
>>>> +                       pte.may_write=true;
>>> The pte struct here is write-only from xlate's point of view. Any 
>>> modification to it doesn't get reflected to the real translation 
>>> layer. But your guess is good, I'd also assume that you're hitting a 
>>> read-only page.
>> Yes, you are right. Thanks for catching this.
>> So, if it is a read-only page, 
>> 'kvmppc_st(vcpu,&pc,sizeof(u32),&new_inst,true)' will not write 
>> anything. Then, there is no reason for guest to hang. Right?
>
> Oh? It should inject a page fault. Maybe it doesn't, but then it 
> behaves badly :).
>
> Either way - just enable the debug tracepoints and check the exits you 
> get right after you hit an mfmsr emulation. That should give you hints 
> on what's going wrong.
I tried a couple of things. But I am still having trouble getting this 
to work.

It seems to me that problem lies in my method of shared page mapping.

Before patching any instruction, shared page needs to be mapped to guest 
effective address. I think, this is all that we need for mapping.

+        struct kvmppc_pte pte;
+        vcpu->arch.mmu.xlate(vcpu,vcpu->arch.magic_page_ea,&pte,true);
+        kvmppc_mmu_map_page(vcpu,&pte);

And, then we can start with patching.

--- a/arch/powerpc/kvm/book3s_emulate.c
+++ b/arch/powerpc/kvm/book3s_emulate.c
@@ -88,6 +88,23 @@ int kvmppc_core_emulate_op(struct kvm_run *run, 
struct kvm_vcpu *vcpu,
                 case OP_31_XOP_MFMSR:
                         kvmppc_set_gpr(vcpu, get_rt(inst),
                                        vcpu->arch.shared->msr);
+
+                       pc = kvmppc_get_pc(vcpu);
+                       //Generate new instruction
+                       addr= (-4096L) + offsetof(struct 
kvm_vcpu_arch_shared, msr);
+                       u32 rt1 = inst & 0x03e00000;
+                       u32 new_inst=0x80000000 | rt1 | ((addr + 4) & 
0x0000fffc);
+
+                       //patch only if magic page is mapped; try 
patching only for 0x7c0000a6 inst
+                       if (vcpu->arch.magic_page_pa=0xfffff000 && 
inst=0x7c0000a6) {
+                                       
kvmppc_st(vcpu,&pc,sizeof(u32),&new_inst,true);
+                                       }
+


Thanks,
Dushyant