linux-kernel - Re: [PATCH 09/11] KVM: MMU: simplify mmu_need_write

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Date:	Tue, 15 Dec 2015 16:43:13 +0800
From:	Kai Huang <kai.huang@...ux.intel.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	pbonzini@...hat.com
Cc:	gleb@...nel.org, mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/11] KVM: MMU: simplify mmu_need_write_protect



On 12/01/2015 02:26 AM, Xiao Guangrong wrote:
> Now, all non-leaf shadow page are page tracked, if gfn is not tracked
> there is no non-leaf shadow page of gfn is existed, we can directly
> make the shadow page of gfn to unsync
>
> Signed-off-by: Xiao Guangrong <guangrong.xiao@...ux.intel.com>
> ---
>   arch/x86/kvm/mmu.c | 26 ++++++++------------------
>   1 file changed, 8 insertions(+), 18 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 5a2ca73..f89e77f 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2461,41 +2461,31 @@ static void __kvm_unsync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
>   	kvm_mmu_mark_parents_unsync(sp);
>   }
>   
> -static void kvm_unsync_pages(struct kvm_vcpu *vcpu,  gfn_t gfn)
> +static bool kvm_unsync_pages(struct kvm_vcpu *vcpu, gfn_t gfn,
> +			     bool can_unsync)
>   {
>   	struct kvm_mmu_page *s;
>   
>   	for_each_gfn_indirect_valid_sp(vcpu->kvm, s, gfn) {
> +		if (!can_unsync)
> +			return true;
How about moving this right before for_each_gfn_indirect_valid_sp? As 
can_unsync is passed as parameter, so there's no point checking it 
several times.

A further thinking is can we move it to mmu_need_write_protect? Passing 
can_unsync as parameter to kvm_unsync_pages sounds a little bit odd.

> +
>   		if (s->unsync)
>   			continue;
>   		WARN_ON(s->role.level != PT_PAGE_TABLE_LEVEL);
How about large page mapping? Such as if guest uses 2M mapping and its 
shadow is indirect, does above WARN_ON still meet? As you removed the PT 
level check in mmu_need_write_protect.

Thanks,
-Kai
>   		__kvm_unsync_page(vcpu, s);
>   	}
> +
> +	return false;
>   }

>   
>   static bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
>   				   bool can_unsync)
>   {
> -	struct kvm_mmu_page *s;
> -	bool need_unsync = false;
> -
>   	if (kvm_page_track_check_mode(vcpu, gfn, KVM_PAGE_TRACK_WRITE))
>   		return true;
>   
> -	for_each_gfn_indirect_valid_sp(vcpu->kvm, s, gfn) {
> -		if (!can_unsync)
> -			return true;
> -
> -		if (s->role.level != PT_PAGE_TABLE_LEVEL)
> -			return true;
> -
> -		if (!s->unsync)
> -			need_unsync = true;
> -	}
> -	if (need_unsync)
> -		kvm_unsync_pages(vcpu, gfn);
> -
> -	return false;
> +	return kvm_unsync_pages(vcpu, gfn, can_unsync);
>   }
>   
>   static bool kvm_is_mmio_pfn(pfn_t pfn)

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