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 12:07:55 +0800
From:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:	Kai Huang <kai.huang@...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 12/17/2015 10:44 AM, Kai Huang wrote:
>
>
> 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.

That is why we get improvement after this patchset, we seep up the case for the write access
happens on non-leaf page tables. ;)

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