linux-kernel - Re: [PATCH v2 6/10] KVM MMU: don't write-protect if have new mapping to unsync page

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Message-ID: <4BD50F63.1010008@cn.fujitsu.com>
Date:	Mon, 26 Apr 2010 11:58:27 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM list <kvm@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 6/10] KVM MMU: don't write-protect if have new mapping
 to unsync page



Avi Kivity wrote:
> On 04/25/2010 10:00 AM, Xiao Guangrong wrote:
>> Two cases maybe happen in kvm_mmu_get_page() function:
>>
>> - one case is, the goal sp is already in cache, if the sp is unsync,
>>    we only need update it to assure this mapping is valid, but not
>>    mark it sync and not write-protect sp->gfn since it not broke unsync
>>    rule(one shadow page for a gfn)
>>
>> - another case is, the goal sp not existed, we need create a new sp
>>    for gfn, i.e, gfn (may)has another shadow page, to keep unsync rule,
>>    we should sync(mark sync and write-protect) gfn's unsync shadow page.
>>    After enabling multiple unsync shadows, we sync those shadow pages
>>    only when the new sp not allow to become unsync(also for the unsyc
>>    rule, the new rule is: allow all pte page become unsync)
>>    
> 
> Another interesting case is to create new shadow pages in the unsync
> state.  That can help when the guest starts a short lived process: we
> can avoid write protecting its pagetables completely.  Even if we do
> sync them, we can sync them in a batch instead of one by one, saving IPIs.

IPI is needed when rmap_write_protect() changes mappings form writable to read-only,
so while we sync all gfn's unsync page, only one IPI is needed.
And, another problem is we call ramp_write_protect()/flush-local-tlb many times when sync gfn's
unsync page, the same problem is in mmu_sync_children() function, could you allow me to improve
it after this patchset? :-)


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