linux-kernel - Re: [PATCH 2/3] KVM: MMU: don not retry #PF for nonpaging guest

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Date:	Mon, 08 Nov 2010 10:14:35 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 2/3] KVM: MMU: don not retry #PF for nonpaging guest

On 11/05/2010 06:31 PM, Gleb Natapov wrote:
> On Fri, Nov 05, 2010 at 04:03:28PM +0800, Xiao Guangrong wrote:
>> On 11/05/2010 03:45 PM, Gleb Natapov wrote:
>>
>>>>
>>>> It looks like something broken: apfs can generated in L2 guest (nested ntp guest)
>>>> and be retried in L1 guest.
>>>>
>>> Why is this a problem? apf will be generate on direct map even when L2
>>> guest is running so it should be OK to prefault it into direct map on
>>> completion.
>>>
>>
>> The nested_cr3 is different between L2 and L1, fix L2's page fault in L1's page table
>> is useless.
> But we are fixing L0 page faults in L0 page table. We do not start apf
> because of L1 faulted in its page table.
> 

Hi Gleb,

For example, NPT Guest L1 runs on Host, and Nested NPT Guest L2 runs on Guest L1.
Now, Guest L2 is running, has below sequences:

a: NPF/PF occurs in L2 Guest, and generates a apf(named A-apf), then
   L2 Guest is blocked

b: a external event wakes up L2 Guest, and let it run again.

c: L2 Guest VMEXIT to L1 Guest because L2 Guest's action is intercepted by Guest L1

d: When cpu enter L1 Guest, A-apf is completed, then it will retry A-apf in
   L1 Guest's mmu context, and this 'retry' is useless.

Could you please point it out for me if i missed something. :-)


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