linux-kernel - Re: [PATCH v2 19/22] KVM: MMU: lockless walking shadow page table

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Date:	Wed, 29 Jun 2011 19:16:39 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v2 19/22] KVM: MMU: lockless walking shadow page table

On 06/29/2011 05:16 PM, Avi Kivity wrote:
> On 06/22/2011 05:35 PM, Xiao Guangrong wrote:
>> Use rcu to protect shadow pages table to be freed, so we can safely walk it,
>> it should run fastly and is needed by mmio page fault
>>
> 
>>   static void kvm_mmu_commit_zap_page(struct kvm *kvm,
>>                       struct list_head *invalid_list)
>>   {
>> @@ -1767,6 +1874,14 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
>>
>>       kvm_flush_remote_tlbs(kvm);
>>
>> +    if (atomic_read(&kvm->arch.reader_counter)) {
>> +        kvm_mmu_isolate_pages(invalid_list);
>> +        sp = list_first_entry(invalid_list, struct kvm_mmu_page, link);
>> +        list_del_init(invalid_list);
>> +        call_rcu(&sp->rcu, free_pages_rcu);
>> +        return;
>> +    }
>> +
> 
> I think we should do this unconditionally.  The cost of ping-ponging the shared cache line containing reader_counter will increase with large smp counts.  On the other hand, zap_page is very rare, so it can be a little slower.  Also, less code paths = easier to understand.
> 

On soft mmu, zap_page is very frequently, it can cause performance regression in my test.
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