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

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Date:	Sat, 03 Jul 2010 15:44:45 +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 v4 5/6] KVM: MMU: combine guest pte read between walk
 and pte prefetch

On 07/03/2010 03:31 PM, Xiao Guangrong wrote:
>
> Avi Kivity wrote:
>
>    
>>>
>>>        
>>          if (!direct) {
>>              r = kvm_read_guest_atomic(vcpu->kvm,
>>                            gw->pte_gpa[level - 2],
>> &curr_pte, sizeof(curr_pte));
>>              if (r || curr_pte != gw->ptes[level - 2]) {
>>                  kvm_mmu_put_page(shadow_page, sptep);
>>                  kvm_release_pfn_clean(pfn);
>>                  sptep = NULL;
>>                  break;
>>              }
>>          }
>>
>> the code you moved... under what scenario is it not sufficient?
>>
>>      
> I not move those code, just use common function instead, that it's
> FNAME(check_level_mapping)(), there are do the same work.
>
> And this check is not sufficient, since it's only checked if the
> mapping is zapped or not exist, for other words only when broken this
> judgment:
> 	is_shadow_present_pte(*sptep)&&  !is_large_pte(*sptep)
>
> but if the middle level is present and it's not the large mapping,
> this check is skipped.
>    


Well, in the description, it looked like everything was using small 
pages (in kvm, level=1 means PTE level, we need to change this one 
day).  Please describe again and say exactly when the guest or host uses 
large pages.


-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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