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

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Date:	Tue, 15 Dec 2015 16:47:23 +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 12/15/2015 04:43 PM, Kai Huang wrote:
>
>
> 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
Btw I also think this patch can be merged with patch 6.

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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