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

Avi Kivity
2011-09-13T10:50:54Z

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.

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