Re: [PATCH 4/4] KVM: MMU: Don't touch unsync sp in kvm_mmu_pte_write()

[Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>]

On 09/20/2010 11:24 PM, Avi Kivity wrote:
>  On 09/20/2010 04:21 PM, Xiao Guangrong wrote:
>> Gfn may have many shadow pages, when one sp need be synced, we write
>> protected sp->gfn and sync this sp but we keep other shadow pages
>> asynchronous
>>
>> So, while gfn happen page fault, let it not touches unsync page, the
>> unsync
>> page only updated at invlpg/flush TLB time
>>
>> @@ -3157,6 +3164,9 @@ void kvm_mmu_pte_write(struct kvm_vcpu *vcpu,
>> gpa_t gpa,
>>
>>       mask.cr0_wp = mask.cr4_pae = mask.nxe = 1;
>>       for_each_gfn_indirect_valid_sp(vcpu->kvm, sp, gfn, node) {
>> +        if (sp->unsync)
>> +            continue;
>> +
>>
> 
> Not sure this is a win.  If a gpte is updated from having p=0 to p=1 (or
> permissions upgraded), we may not have an invlpg to sync the spte, since
> the hardware doesn't require it.  With this change, we may get an extra
> #PF.
> 

Avi,

Thanks for your review, i think this case is not too bad since:

1: This case only impacts local vcpu since if permissions is increased, it's 
   no need send IPT to flush remote vcpu's tlb, so even if we update unsync
   spte in kvm_mmu_pte_write() path, the #PF still occur on other vcpus. 

2: If the unsync sp which is updated in kvm_mmu_pte_write() is not using by the
   vcpu, it will sync automatically after it's loaded.

3: If the sp is using, update this sp in kvm_mmu_pte_write() will avoid extra #PF,
   in this case, two(or more) sps have the same gfn, there are mapped in the same
   page table and with different kinds(unsync/sync), i thinks this case is infrequency.
   And even we updated it, we can not sure it can be accessed latter, 

So, i think it's better lazily update unsync sp until it's used or the flush time,
your opinion? :-)