linux-kernel - Re: [PATCH v2 1/10] KVM: MMU: fix writable sync sp mapping

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Date:	Mon, 28 Jun 2010 14:41:34 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 1/10] KVM: MMU: fix writable sync sp mapping

On 06/28/2010 12:40 PM, Xiao Guangrong wrote:
>
> Avi Kivity wrote:
>
>    
>>>        for_each_gfn_indirect_valid_sp(vcpu->kvm, s, gfn, node) {
>>> +        if (!can_unsync)
>>> +            return 1;
>>> +
>>>
>>>        
>> What if the page is already unsync?  We don't need write protection in
>> this case.
>>      
> Avi,
>
> The reason is when we sync children sps, we write-protected for all sps first,
> list relevant code:
>
> | static void mmu_sync_children(...)
> | {
> |	......
> |		for_each_sp(pages, sp, parents, i)
> |			protected |= rmap_write_protect(vcpu->kvm, sp->gfn);<==== A
> |
> |		if (protected)
> |			kvm_flush_remote_tlbs(vcpu->kvm);
> |
> |		for_each_sp(pages, sp, parents, i) {
> |			kvm_sync_page(vcpu, sp,&invalid_list);<==== B
> |			mmu_pages_clear_parents(&parents);
> |		}
> |	......
> |}
>
> For example:
>
> SP1.pte[0] = P
> SP2.gfn's pfn = P
> [SP1.pte[0] = SP2.gfn's pfn]
>
> At A point, SP1.gfn and SP2.gfn are write-protected.
>
> At B point, if sync SP1 first, while it's synced. it will detect SP1.pte[0].gfn only has one unsync-sp,
> that is SP2, so it will mapping it writable, then we sync SP2, we will set SP2 to sync page.
>
> The final result is: SP2 is the sync page but SP2.gfn is writable.
>    


I think I see.  So, after A, the pages are write protected, but are 
still marked as unsync.  In B, we're testing SP2->unsync, which we plan 
to sync soon, but haven't yet.  So the test for s->unsync is incorrect.

So the patch is right.  Thanks for the explanation.  Please update the 
changelog to note that sp->unsync is not reliable during resync, this is 
tricky stuff.

-- 
error compiling committee.c: too many arguments to function

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