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 15:49:29 +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 03:44 PM, Avi Kivity wrote:
> 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.
>
>

Oh, I see what you mean.

Regarding the patch, is it possible just to move the check before, 
instead of adding the 'check' variable?

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

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