linux-kernel - Re: [PATCH 08/12] KVM: MMU: move zap/flush to kvm_mmu_get

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Date:	Thu, 25 Feb 2016 15:32:49 +0800
From:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc:	yoshikawa_takuya_b1@....ntt.co.jp, mtosatti@...hat.com
Subject: Re: [PATCH 08/12] KVM: MMU: move zap/flush to kvm_mmu_get_page

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@...hat.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.