linux-kernel - Re: [PATCH v6 2/7] KVM: MMU: delete shadow page from hash list in kvm_mmu_prepare_zap

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Date:	Mon, 20 May 2013 17:19:26 +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 v6 2/7] KVM: MMU: delete shadow page from hash list in
 kvm_mmu_prepare_zap_page

On 05/19/2013 06:47 PM, Gleb Natapov wrote:
> On Fri, May 17, 2013 at 05:12:57AM +0800, Xiao Guangrong wrote:
>> Move deletion shadow page from the hash list from kvm_mmu_commit_zap_page to
>> kvm_mmu_prepare_zap_page so that we can call kvm_mmu_commit_zap_page
>> once for multiple kvm_mmu_prepare_zap_page that can help us to avoid
>> unnecessary TLB flush
>>
> Don't we call kvm_mmu_commit_zap_page() once for multiple
> kvm_mmu_prepare_zap_page() now when possible? kvm_mmu_commit_zap_page()
> gets a list as a parameter. I am not against the change, but wish to
> understand it better.

The changelong is not clear enough, i mean we can "call
kvm_mmu_commit_zap_page once for multiple kvm_mmu_prepare_zap_page" when
we use lock-break technique. If we do not do this, the page can be found
in hashtable but they are linked on the invalid_list on other thread.

> 
>> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
>> ---
>>  arch/x86/kvm/mmu.c |    8 ++++++--
>>  1 files changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>> index 40d7b2d..682ecb4 100644
>> --- a/arch/x86/kvm/mmu.c
>> +++ b/arch/x86/kvm/mmu.c
>> @@ -1466,7 +1466,7 @@ static inline void kvm_mod_used_mmu_pages(struct kvm *kvm, int nr)
>>  static void kvm_mmu_free_page(struct kvm_mmu_page *sp)
>>  {
>>  	ASSERT(is_empty_shadow_page(sp->spt));
>> -	hlist_del(&sp->hash_link);
>> +
>>  	list_del(&sp->link);
>>  	free_page((unsigned long)sp->spt);
>>  	if (!sp->role.direct)
>> @@ -1655,7 +1655,8 @@ static void kvm_mmu_commit_zap_page(struct kvm *kvm,
>>  
>>  #define for_each_gfn_indirect_valid_sp(_kvm, _sp, _gfn)			\
>>  	for_each_gfn_sp(_kvm, _sp, _gfn)				\
>> -		if ((_sp)->role.direct || (_sp)->role.invalid) {} else
>> +		if ((_sp)->role.direct ||				\
>> +		      ((_sp)->role.invalid && WARN_ON(1))) {} else
>>  
>>  /* @sp->gfn should be write-protected at the call site */
>>  static int __kvm_sync_page(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>> @@ -2074,6 +2075,9 @@ static int kvm_mmu_prepare_zap_page(struct kvm *kvm, struct kvm_mmu_page *sp,
>>  		unaccount_shadowed(kvm, sp->gfn);
>>  	if (sp->unsync)
>>  		kvm_unlink_unsync_page(kvm, sp);
>> +
>> +	hlist_del_init(&sp->hash_link);
>> +
> What about moving this inside if() bellow and making it hlist_del()?
> Leave the page on the hash if root_count is non zero.
> 

It's a good idea. will update.


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