Re: [PATCH v7 10/11] KVM: MMU: collapse TLB flushes when zap allpages

From: Xiao Guangrong
Date: Thu May 23 2013 - 03:39:07 EST

On 05/23/2013 03:37 PM, Xiao Guangrong wrote:
> On 05/23/2013 03:24 PM, Gleb Natapov wrote:
>> 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()?
> After call kvm_flush_remote_tlbs(), the page still exists on vcpu->root,

Sorry, should be kvm_reload_remote_mmus() here.

> so we can not protect the page is being used by other vcpu.
> But before call kvm_mmu_commit_zap_page(), the page has been deleted from
> vcpu's page table, after call kvm_flush_remote_tlbs(), we can ensure that
> other vcpus can not find these pages.

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