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

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Date:	Wed, 16 Dec 2015 15:51:14 +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 12/15/2015 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? 
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.

Thanks,
-Kai
>
>>>   /*
>>>    * remove the guest page from the tracking pool which stops the 
>>> interception
>>>    * of corresponding access on that page. It is the opposed 
>>> operation of
>>> @@ -134,20 +160,12 @@ void kvm_page_track_remove_page(struct kvm 
>>> *kvm, gfn_t gfn,
>>>       struct kvm_memory_slot *slot;
>>>       int i;
>>> -    WARN_ON(!check_mode(mode));
>>> -
>>>       for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
>>>           slots = __kvm_memslots(kvm, i);
>>>           slot = __gfn_to_memslot(slots, gfn);
>>>           spin_lock(&kvm->mmu_lock);
>>> -        update_gfn_track(slot, gfn, mode, -1);
>>> -
>>> -        /*
>>> -         * allow large page mapping for the tracked page
>>> -         * after the tracker is gone.
>>> -         */
>>> -        kvm_mmu_gfn_allow_lpage(slot, gfn);
>>> +        kvm_slot_page_track_remove_page_nolock(kvm, slot, gfn, mode);
>> Looks you need to merge this part with patch 1, as you are modifying
>> kvm_page_track_{add,remove}_page here, which are introduced in your 
>> patch 1.
>
> Indeed, it is better.
>
>

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