Re: [PATCH v3 01/11] KVM: MMU: avoid pte_list_desc running out in kvm_mmu_pte_write

Xiao Guangrong
2011-09-13T19:29:20Z

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. :-)