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From: Wanpeng Li <kernellwp@gmail.com>
Date: Thu, 11 May 2017 21:49:07 +0800
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Subject: Re: [Qemu-devel] About QEMU BQL and dirty log switch in Migration
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To: "Zhoujian (jay)" <jianjay.zhou@huawei.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>, yanghongyang <yanghongyang@huawei.com>, "quintela@redhat.com" <quintela@redhat.com>, "wangxin (U)" <wangxinxin.wang@huawei.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, "Gonglei (Arei)" <arei.gonglei@huawei.com>, Huangzhichao <huangzhichao@huawei.com>, Zhanghailiang <zhang.zhanghailiang@huawei.com>, "Herongguang (Stephen)" <herongguang.he@huawei.com>, Xiao Guangrong <xiaoguangrong@tencent.com>, Paolo Bonzini <pbonzini@redhat.com>

2017-05-11 21:43 GMT+08:00 Wanpeng Li <kernellwp@gmail.com>:
> 2017-05-11 20:24 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
>>
>>
>> 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);
>
> This is an unlikely branch(unless guest live migration fails and
> continue to run on the source machine) instead of hot path, do you
> have any performance number for your real workloads?

I find the original discussion by google.
https://lists.nongnu.org/archive/html/qemu-devel/2017-04/msg04143.html
You will not go to this branch if the guest live migration
successfully.

Regards,
Wanpeng Li