Re: Patching guest kernel code for better performance from HOST

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

On Tuesday 05 April 2011 02:39 PM, Alexander Graf wrote:
> On 05.04.2011, at 10:59, Dushyant Bansal wrote:
>
>    
>> Hi all,
>>
>>     I understand that in order to improve performance by reducing vm exits, kvm uses one shared page between kvm and guest. Now, guest tells the host to map the magic page to '-4096'.
>>
>> How does kvm make sure that guest will not use this address ( -4096) for other purpose?
>>
>> Then, guest itself patches its kernel.
>> I am trying to patch guest from host. So far, I have tried to patch MFMSR instruction. Patching is done when kvm tries to emulate MFMSR. Mechanism used to generate new instruction is same as used by guest in "arch/powerpc/kernel/kvm.c"
>>
>> --- 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?

--
Dushyant