Re: [PATCH 2/3] KVM: MMU: don not retry #PF for nonpaging guest

[Xiao Guangrong]

On 11/09/2010 06:51 PM, Gleb Natapov wrote:
> On Tue, Nov 09, 2010 at 05:52:40PM +0800, Xiao Guangrong wrote:
>>>>>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>>>>>> index 7f20f2c..606978e 100644
>>>>>> --- a/arch/x86/include/asm/kvm_host.h
>>>>>> +++ b/arch/x86/include/asm/kvm_host.h
>>>>>> @@ -600,6 +600,7 @@ struct kvm_x86_ops {
>>>>>>  struct kvm_arch_async_pf {
>>>>>>  	u32 token;
>>>>>>  	gfn_t gfn;
>>>>>> +	bool softmmu;
>>>>>>  };
>>>>>>  
>>>>>>  extern struct kvm_x86_ops *kvm_x86_ops;
>>>>>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>>>>>> index f3fad4f..48ca312 100644
>>>>>> --- a/arch/x86/kvm/mmu.c
>>>>>> +++ b/arch/x86/kvm/mmu.c
>>>>>>  static int kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu, gva_t gva, gfn_t gfn)
>>>>>> @@ -2602,6 +2607,7 @@ static int kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu, gva_t gva, gfn_t gfn)
>>>>>>  	struct kvm_arch_async_pf arch;
>>>>>>  	arch.token = (vcpu->arch.apf.id++ << 12) | vcpu->vcpu_id;
>>>>>>  	arch.gfn = gfn;
>>>>>> +	arch.softmmu = mmu_is_softmmu(vcpu);
>>>>>>  
>>>>> We can do:
>>>>>         if (mmu_is_nested(vcpu))
>>>>> 		gva = vcpu->mmu.gva_to_gpa(gva);
>>>>> And this should fix everything no?
>>>>>
>>>>
>>>> No, since it can't help us to avoid NPF when nested guest run again.
>>>>
>>> Of course it will not prevent NPF if L2 guest touches it again, but from
>>> correctness point of view it is OK. So if L1 will re-use the page for
>>> L1 process the page will be already mapped. Not a huge gain I agree, but
>>> fix is much more simple.
>>>
>>
>> Um, it need hold mmu_lock, and we don't know 'gva''s mapping in PT10 is valid
>> or not, also don't know whether it can be accessed later, so the general rule
>> is lazily update it. 
>>
> We do know that gva's mapping in PT10 is valid since we wouldn't try apf
> otherwise. If nested gpa is mapped to a gpa thst is not valid in L0 then
> L0 should emulate instruction for L2, no?
> 

No need.

>> The more important is that we can prefault for softmmu in the later patch,
>> it means we can prefault 'gva' in PT20, so don't cook gva here.
>>
> So may be just apply second patch then?
> 

Yes, i think so.
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