linux-kernel - Re: [PATCH v7 10/11] KVM: MMU: collapse TLB flushes when zap all pages

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Date:	Thu, 23 May 2013 14:26:57 +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 v7 10/11] KVM: MMU: collapse TLB flushes when zap all
 pages

On 05/23/2013 02:12 PM, Gleb Natapov wrote:
> On Thu, May 23, 2013 at 03:55:59AM +0800, Xiao Guangrong wrote:
>> kvm_zap_obsolete_pages uses lock-break technique to zap pages,
>> it will flush tlb every time when it does lock-break
>>
>> We can reload mmu on all vcpus after updating the generation
>> number so that the obsolete pages are not used on any vcpus,
>> after that we do not need to flush tlb when obsolete pages
>> are zapped
>>
>> Note: kvm_mmu_commit_zap_page is still needed before free
>> the pages since other vcpus may be doing locklessly shadow
>> page walking
>>
> Since obsolete pages are not accessible for lockless page walking after
> reload of all roots I do not understand why additional tlb flush is

kvm_reload_remote_mmus() forces vcpus to leave guest mode, but if the
vcpu is not running on guest mode, it does nothing except set the request
bit. So, the vcpu can go lockless page walking after kvm_reload_remote_mmus()
return on other vcpu.

Like this scenario:

VCPU 0                              VCPU 1
                                 exit when it encounters #PF

kvm_reload_remote_mmus(){
    set vcpu1->request bit;

    do not send IPI due to
    vcpu 1 not running on guest mode

                                 call page-fault handler then go lockless walking !!!
    return
}


> needed. Also why tlb flush should prevent lockless-walking from using
> the page? Making page unreachable from root_hpa does that, no?

lockless-walking disables the interrupt and makes the vcpu state as
READING_SHADOW_PAGE_TABLES, this state is treated as GUEST_MODE,
kvm_flush_remote_tlbs() should send IPI to this vcpu in this case.

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