Re: [Qemu-devel] About QEMU BQL and dirty log switch in Migration

[Xiao Guangrong <guangrong.xiao@gmail.com>]

On 05/11/2017 08:24 PM, Paolo Bonzini wrote:
> 
> 
> On 11/05/2017 14:07, Zhoujian (jay) wrote:
>> -        * Scan sptes if dirty logging has been stopped, dropping those
>> -        * which can be collapsed into a single large-page spte.  Later
>> -        * page faults will create the large-page sptes.
>> +        * Reset each vcpu's mmu, then page faults will create the large-page
>> +        * sptes later.
>>           */
>>          if ((change != KVM_MR_DELETE) &&
>>                  (old->flags & KVM_MEM_LOG_DIRTY_PAGES) &&
>> -               !(new->flags & KVM_MEM_LOG_DIRTY_PAGES))
>> -               kvm_mmu_zap_collapsible_sptes(kvm, new);
>> +               !(new->flags & KVM_MEM_LOG_DIRTY_PAGES)) {
>> +               kvm_for_each_vcpu(i, vcpu, kvm)
>> +                       kvm_mmu_reset_context(vcpu);
> 
> This should be "kvm_make_all_cpus_request(kvm, KVM_REQ_MMU_RELOAD);" but
> I am not sure it is enough.  I think that if you do not zap the SPTEs,
> the page faults will use 4K SPTEs, not large ones (though I'd have to
> check better; CCing Xiao and Wanpeng).

Yes, Paolo is right. kvm_mmu_reset_context() just reloads vCPU's
root page table, 4k mappings are still kept.

There are two issues reported:
- one is kvm_mmu_slot_apply_flags(), when enable dirty log tracking.

   Its root cause is kvm_mmu_slot_remove_write_access() takes too much
   time.

   We can make the code adaptive to use the new fast-write-protect faculty
   introduced by my patchset, i.e, if the number of pages contained in this
   memslot is more than > TOTAL * FAST_WRITE_PROTECT_PAGE_PERCENTAGE, then
   we use fast-write-protect instead.

- another one is kvm_mmu_zap_collapsible_sptes() when disable dirty
   log tracking.

   collapsible_sptes zaps 4k mappings to make memory-read happy, it is not
   required by the semanteme of KVM_SET_USER_MEMORY_REGION and it is not
   urgent for vCPU's running, it could be done in a separate thread and use
   lock-break technology.

Thanks!