linux-kernel - Re: [PATCH 01/11] KVM: MMU: avoid pte_list_desc run out in kvm_mmu_pte

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Date:	Wed, 27 Jul 2011 17:37:13 +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 01/11] KVM: MMU: avoid pte_list_desc run out in kvm_mmu_pte_write

On 07/27/2011 05:00 PM, Avi Kivity wrote:
> On 07/26/2011 02:25 PM, Xiao Guangrong wrote:
>> kvm_mmu_pte_write is unsafe since we need to alloc pte_list_desc in the
>> function when spte is prefetched, unfortunately, we can not know how many
>> spte need to be prefetched on this path, that means we can use out of the
>> free  pte_list_desc object in the cache, and BUG_ON() is triggered, also some
>> path does not fill the cache, such as INS instruction emulated that does not
>> trigger page fault
>>
>>
>>   void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
>>                  const u8 *new, int bytes,
>>                  bool guest_initiated)
>> @@ -3583,6 +3596,7 @@ void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
>>           break;
>>       }
>>
>> +    mmu_topup_memory_caches(vcpu);
> 
> Please add a comment here describing why it's okay to ignore the error return.
> 

OK

>>       spin_lock(&vcpu->kvm->mmu_lock);
>>       if (atomic_read(&vcpu->kvm->arch.invlpg_counter) != invlpg_counter)
>>           gentry = 0;
>> @@ -3653,7 +3667,7 @@ void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
>>               mmu_page_zap_pte(vcpu->kvm, sp, spte);
>>               if (gentry&&
>>               !((sp->role.word ^ vcpu->arch.mmu.base_role.word)
>> -            &  mask.word))
>> +            &  mask.word)&&  get_free_pte_list_desc_nr(vcpu))
>>                   mmu_pte_write_new_pte(vcpu, sp, spte,&gentry);
> 
> Wow, this bug was here since 2.6.23.  Good catch.
> 
> Please wrap or rename get_free_pte_list_desc_nr() in rmap_can_add(vcpu) so it's clearer why we're doing this.
> 

OK, will do, thanks!

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