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: <4E6FAFA3.4010108@cn.fujitsu.com>
Date:	Wed, 14 Sep 2011 03:31:47 +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 06:50 PM, Avi Kivity wrote:
> On 09/13/2011 01:24 PM, Xiao Guangrong wrote:
>> 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?
>>
> 
> Yes.
> 
> btw, is rmap_can_add() sufficent?  We allocate more than just rmaps in mmu_topup_memory_caches().  I guess it is, but this is getting tricky.
> 

rmap_can_add() is used to avoid prefetching sptes more than the number of rmaps in
the cache, because we do not know the exact number of sptes to be fetched. :-)


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