linux-kernel - Re: [PATCH 08/11] KVM: MMU: use page track for non-leaf shadow pages

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Date:	Thu, 17 Dec 2015 10:44:12 +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 08/11] KVM: MMU: use page track for non-leaf shadow pages



On 12/16/2015 04:39 PM, Xiao Guangrong wrote:
>
>
> On 12/16/2015 03:51 PM, Kai Huang wrote:
>>
>>
>> On 12/15/2015 05:10 PM, Xiao Guangrong wrote:
>>>
>>>
>>> On 12/15/2015 03:52 PM, Kai Huang wrote:
>>>
>>>>>   static bool __mmu_gfn_lpage_is_disallowed(gfn_t gfn, int level,
>>>>> @@ -2140,12 +2150,18 @@ static struct kvm_mmu_page 
>>>>> *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
>>>>>       hlist_add_head(&sp->hash_link,
>>>>> &vcpu->kvm->arch.mmu_page_hash[kvm_page_table_hashfn(gfn)]);
>>>>>       if (!direct) {
>>>>> -        if (rmap_write_protect(vcpu, gfn))
>>>>> +        /*
>>>>> +         * we should do write protection before syncing pages
>>>>> +         * otherwise the content of the synced shadow page may
>>>>> +         * be inconsistent with guest page table.
>>>>> +         */
>>>>> +        account_shadowed(vcpu->kvm, sp);
>>>>> +
>>>>> +        if (level == PT_PAGE_TABLE_LEVEL &&
>>>>> +              rmap_write_protect(vcpu, gfn))
>>>>>               kvm_flush_remote_tlbs(vcpu->kvm);
>>>> I think your modification is good but I am little bit confused 
>>>> here. In account_shadowed, if
>>>> sp->role.level > PT_PAGE_TABLE_LEVEL, the sp->gfn is write 
>>>> protected, and this is reasonable. So why
>>>> write protecting the gfn of PT_PAGE_TABLE_LEVEL here?
>>>
>>> Because the shadow page will become 'sync' that means the shadow 
>>> page will be synced
>>> with the page table in guest. So the shadow page need to be 
>>> write-protected to avoid
>>> the guest page table is changed when we do the 'sync' thing.
>>>
>>> The shadow page need to be write-protected to avoid that guest page 
>>> table is changed
>>> when we are syncing the shadow page table. See kvm_sync_pages() 
>>> after doing
>>> rmap_write_protect().
>> I see. So why are you treat PT_PAGE_TABLE_LEVEL gfn separately here? 
>> why this cannot be done in
>> account_shadowed, as you did for upper level sp?
>
> non-leaf shadow pages are keepking write-protected which page fault 
> handler can not fix write
> access on it. And leaf shadow pages are not.
My point is the original code didn't separate the two cases so I am not 
sure why you need to separate. Perhaps you want to make account_shadowed 
imply the non-leaf guest page table is write-protected while leaf page 
table is not.

Thanks,
-Kai
>> Actually I am thinking whether account_shadowed is
>> overdoing things. From it's name it should only *account* shadow sp, 
>> but now it also does write
>> protection and disable large page mapping.
>>
>
> Hmm.. disable large page mapping is already in current code... i think 
> account_shadowed() can
> be understood as new page is taken into account, so protection things 
> are needed there.
>
> But I am not good at naming function and also my english is not good 
> enough, any other better name
> is welcome. ;)
>
>

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