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 16:39:43 +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 12/16/2015 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.

> 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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