linux-kernel - Re: [PATCH v3 01/11] KVM: MMU: avoid pte_list_desc running out in kvm_mmu_pte

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Message-ID: <4E6F2F54.4030004@cn.fujitsu.com>
Date:	Tue, 13 Sep 2011 18:24:20 +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 v3 01/11] KVM: MMU: avoid pte_list_desc running out in
 kvm_mmu_pte_write

On 09/13/2011 05:51 PM, Avi Kivity wrote:
> On 08/30/2011 05:34 AM, 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
>>
>> @@ -3716,10 +3735,6 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gva_t cr2, u32 error_code,
>>           goto out;
>>       }
>>
>> -    r = mmu_topup_memory_caches(vcpu);
>> -    if (r)
>> -        goto out;
>> -
>>       er = x86_emulate_instruction(vcpu, cr2, 0, insn, insn_len);
>>
> 
> Suppose we are out of memory, can't this get us in an endless loop?
> 
> return -ENOMEM breaks as out (and kills the guest, likely).
> 

If memory is not enough, we just clear sptes on pte_write path(not prefetch spte),
the later page fault path can return -1 to let guest crash. Hmm?

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