Re: [PATCH 11/11] KVM: MMU: improve write flooding detected

[Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>]

On 08/25/2011 10:04 AM, Marcelo Tosatti wrote:

>>> Yes, in this case, the sp is not zapped, but it is hardly to know the gfn
>>> is not used as gpte just depends on writing, for example, the guest can
>>> change the mapping address or the status bit, and so on...The sp can be
>>> zapped if the guest write it again(on the same address), i think it is
>>> acceptable, anymore, it is just the speculative way to zap the unused
>>> shadow page...your opinion?
>>
>> It could increase the flood count independently of the accessed bit of
>> the spte being updated, zapping after 3 attempts as it is now.
>>
>> But additionally reset the flood count if the gpte appears to be valid
>> (points to an existant gfn if the present bit is set, or if its zeroed).
> 
> Well not zero, as thats a common pattern for non ptes.
> 

Hi Marcelo,

Maybe it is not good i think, for some reasons:
- checking gfn valid which it is pointed by gpte is high overload,
  it needs to call gfn_to_hva to walk memslots, especially. kvm_mmu_pte_write
  is called very frequently on shadow mmu.

- MMIO gfn is not an existent gfn, but it is valid pointed by gpte

- we can check the reserved bits in the gpte to check whether it is valid a
  gpte, but for some paging modes, all bits are valid.(for example, non-PAE mode)

- it can not work if the gfn has multiple shadow pages, for example:
  if the gfn was used as PDE, later it is used as PTE, then we have two shadow
  pages: sp1.level = 2, sp2.level = 1, sp1 can not be zapped even even though it
  is not used anymore.

- sometime, we need to zap the shadow page even though the gpte is written validly:
  if the gpte is written frequently but infrequently accessed, we do better zap the
  shadow page to let it is writable(write it directly without #PF) and map it when it
  is accessed, one example is from Avi, the guest OS may update many gptes at one time
  after one page fault.