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

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Message-ID: <20130523072451.GD26157@redhat.com>
Date:	Thu, 23 May 2013 10:24:51 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.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 Thu, May 23, 2013 at 02:26:57PM +0800, Xiao Guangrong wrote:
> 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.

kvm_flush_remote_tlbs() uses the same make_all_cpus_request() as
kvm_reload_remote_mmus() does, so why the same scenario you describe
above cannot happen with kvm_flush_remote_tlbs()?

--
			Gleb.
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