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Re: [PATCH 08/12] KVM: MMU: move zap/flush to kvm_mmu_get_page

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On 02/24/2016 09:17 PM, Paolo Bonzini wrote:
> kvm_mmu_get_page is the only caller of kvm_sync_page_transient
> and kvm_sync_pages.  Moving the handling of the invalid_list there
> removes the need for the underdocumented kvm_sync_page_transient
> function.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> 	Guangrong, at this point I am confused about why
> 	kvm_sync_page_transient didn't clear sp->unsync.  Do
> 	you remember?  Or perhaps kvm_mmu_get_page could just
> 	call kvm_sync_page now?
>

It is the optimization to reduce write-protect as changing unsync to
sync need to write-protect the page and sync all sptes pointing to the
same gfn.

However, after syncing the content between unsync-ed spte and guest pte,
we can reuse this spte perfectly.

> 	Also, can you explain the need_sync variable in
> 	kvm_mmu_get_page?

This is because we need to to protect the semanteme of 'unsync spte' as
only the spte on last level (level = 1) can be unsync so that if a spte
on the upper level is created we should eliminate all the unsync sptes
pointing to the same gfn.

As you have already merged this patchset to the kvm tree, i will post
a patch to comment these cases to make the code be more understandable.

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[PATCH 00/12] KVM: MMU: cleanup around kvm_sync_page, and a few micro-optimizations Paolo Bonzini <pbonzini@redhat.com> - 2016-02-24 14:20 +0100
  [PATCH 10/12] KVM: mark memory barrier with smp_mb__after_atomic Paolo Bonzini <pbonzini@redhat.com> - 2016-02-24 14:20 +0100
  [PATCH 05/12] KVM: MMU: use kvm_sync_page in kvm_sync_pages Paolo Bonzini <pbonzini@redhat.com> - 2016-02-24 14:30 +0100
  [PATCH 08/12] KVM: MMU: move zap/flush to kvm_mmu_get_page Paolo Bonzini <pbonzini@redhat.com> - 2016-02-24 14:30 +0100
    Re: [PATCH 08/12] KVM: MMU: move zap/flush to kvm_mmu_get_page Xiao Guangrong <guangrong.xiao@linux.intel.com> - 2016-02-25 08:40 +0100
      Re: [PATCH 08/12] KVM: MMU: move zap/flush to kvm_mmu_get_page Paolo Bonzini <pbonzini@redhat.com> - 2016-02-25 09:50 +0100
  [PATCH 02/12] KVM: MMU: check kvm_mmu_pages and mmu_page_path indices Paolo Bonzini <pbonzini@redhat.com> - 2016-02-24 14:30 +0100
  [PATCH 03/12] KVM: MMU: introduce kvm_mmu_flush_or_zap Paolo Bonzini <pbonzini@redhat.com> - 2016-02-24 14:30 +0100
  [PATCH 04/12] KVM: MMU: move TLB flush out of __kvm_sync_page Paolo Bonzini <pbonzini@redhat.com> - 2016-02-24 14:30 +0100
  Re: [PATCH 00/12] KVM: MMU: cleanup around kvm_sync_page, and a few  micro-optimizations Xiao Guangrong <guangrong.xiao@linux.intel.com> - 2016-02-25 09:30 +0100
    Re: [PATCH 00/12] KVM: MMU: cleanup around kvm_sync_page, and a few  micro-optimizations Paolo Bonzini <pbonzini@redhat.com> - 2016-02-25 09:50 +0100

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