Re: [PATCH v4 5/6] KVM: MMU: combine guest pte read between walk and pte prefetch

[Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>]

Avi Kivity wrote:
> On 07/03/2010 01:31 PM, Xiao Guangrong wrote:
>>
>>> See how the pte is reread inside fetch with mmu_lock held.
>>>
>>>      
>> It looks like something is broken in 'fetch' functions, this patch will
>> fix it.
>>
>> Subject: [PATCH] KVM: MMU: fix last level broken in FNAME(fetch)
>>
>> We read the guest level out of 'mmu_lock', sometimes, the host mapping is
>> confusion. Consider this case:
>>
>> VCPU0:                                              VCPU1
>>
>> Read guest mapping, assume the mapping is:
>> GLV3 ->  GLV2 ->  GLV1 ->  GFNA,
>> And in the host, the corresponding mapping is
>> HLV3 ->  HLV2 ->  HLV1(P=0)
>>
>>                                                     Write GLV1 and
>> cause the
>>                                                     mapping point to GFNB
>>                                                     (May occur in
>> pte_write or
>>                                                        invlpg path)
>>
>> Mapping GLV1 to GFNA
>>
>> This issue only occurs in the last indirect mapping, since if the middle
>> mapping is changed, the mapping will be zapped, then it will be detected
>> in the FNAME(fetch) path, but when it map the last level, it not checked.
>>
>> Fixed by also check the last level.
>>
>>    
> 
> I don't really see what is fixed.  We already check the gpte.  What's
> special about the new scenario?
> 

I mean is: while we map the last level, we will directly set to the pfn but
the pfn is got by walk_addr, at this time, the guest mapping may be changed.

What is the 'We already check the gpte' mean? i think i miss something :-(