linux-kernel - Re: [PATCH v8 00/11] KVM: MMU: fast zap all shadow pages

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Date:	Sun, 09 Jun 2013 17:06:24 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	avi.kivity@...il.com, mtosatti@...hat.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v8 00/11] KVM: MMU: fast zap all shadow pages

On 06/09/2013 04:53 PM, Gleb Natapov wrote:
> On Fri, May 31, 2013 at 08:36:19AM +0800, Xiao Guangrong wrote:
>> Hi Gleb, Paolo, Marcelo,
>>
>> I have putted the potential controversial patches to the latter that are
>> patch 8 ~ 10, patch 11 depends on patch 9. Other patches are fully reviewed,
>> I think its are ready for being merged. If not luck enough, further discussion
>> is needed, could you please apply that patches first? :)
>>
>> Thank you in advance!
>>
>> Some points are raised during discussion but missed in this version:
>> 1) Gleb's idea that skip obsolete pages in the hast list walker
>>
>>    Unfortunately, it is not safe. There has a window between updating
>>    valid-gen and reloading mmu, in that window, the obsolete page can
>>    be used by vcpu, but the guest page table fail to be write-protected
>>    (since the obsolete page is skipped in mmu_need_write_protect()).
>>
> Can you elaborate on how this can happen. valid_gen is updated under
> mmu_lock and reloading of mmus happens under the same lock, so for all
> other vcpus this should look like atomic thing.

You're right.

Actually, i made another optimization patch in this version that moves
kvm_reload_remote_mmus() out of mmu-lock, but did not attach it into this
series. It seems my brain is not parallel-able enough. :(

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