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

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Message-ID: <4E2FD3AC.1000701@redhat.com>
Date:	Wed, 27 Jul 2011 12:00:28 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.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 07/26/2011 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.

>   	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.


-- 
error compiling committee.c: too many arguments to function

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