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    SubjectRe: [PATCH 11/11] KVM: MMU: improve write flooding detected
    On Thu, Aug 25, 2011 at 03:57:22PM +0800, Xiao Guangrong wrote:
    > On 08/24/2011 03:09 AM, Marcelo Tosatti wrote:
    > > On Wed, Aug 24, 2011 at 12:32:32AM +0800, Xiao Guangrong wrote:
    > >> On 08/23/2011 08:38 PM, Marcelo Tosatti wrote:
    > >>
    > >>>> And, i think there are not problems since: if the spte without accssed bit is
    > >>>> written frequently, it means the guest page table is accessed infrequently or
    > >>>> during the writing, the guest page table is not accessed, in this time, zapping
    > >>>> this shadow page is not bad.
    > >>>
    > >>> Think of the following scenario:
    > >>>
    > >>> 1) page fault, spte with accessed bit is created from gpte at gfnA+indexA.
    > >>> 2) write to gfnA+indexA, spte has accessed bit set, write_flooding_count
    > >>> is not increased.
    > >>> 3) repeat
    > >>>
    > >>
    > >> I think the result is just we hoped, we do not want to zap the shadow page
    > >> because the spte is currently used by the guest, it also will be used in the
    > >> next repetition. So do not increase 'write_flooding_count' is a good choice.
    > >
    > > Its not used. Step 2) is write to write protected shadow page at
    > > gfnA.
    > >
    > >> Let's consider what will happen if we increase 'write_flooding_count':
    > >> 1: after three repetitions, zap the shadow page
    > >> 2: in step 1, we will alloc a new shadow page for gpte at gfnA+indexA
    > >> 3: in step 2, the flooding count is creased, so after 3 repetitions, the
    > >> shadow page can be zapped again, repeat 1 to 3.
    > >
    > > The shadow page will not be zapped because the spte created from
    > > gfnA+indexA has the accessed bit set:
    > >
    > > if (spte && !(*spte & shadow_accessed_mask))
    > > sp->write_flooding_count++;
    > > else
    > > sp->write_flooding_count = 0;
    > >
    >
    > Marcelo, i am still confused with your example, in step 3), what is repeated?
    > it repeats step 2) or it repeats step 1) and 2)?
    >
    > Only step 2) is repeated i guess, right? if it is yes, it works well:
    > when the guest writes gpte, the spte of corresponding shadow page is zapped
    > (level > 1) or it is speculatively fetched(level == 1), the accessed bit is
    > cleared in both case.

    Right.

    > the later write can detect that the accessed bit is not set, and write_flooding_count
    > is increased. finally, the shadow page is zapped, the gpte is written directly.
    >
    > >> The result is the shadow page for gfnA is alloced and zapped again and again,
    > >> yes?
    > >
    > > The point is you cannot rely on the accessed bit of sptes that have been
    > > instantiated with the accessed bit set to decide whether or not to zap.
    > > Because the accessed bit will only be cleared on host memory pressure.
    > >
    >
    > But the accessed bit is also cleared after spte is written.

    Right. But only one of the 512 sptes. Worst case, a shadow that has 1
    spte with accessed bit at every 3 spte entries would not be zapped for a
    linear write of the entire guest pagetable. The current heuristic does
    not suffer from this issue.

    I guess it is OK to be more trigger happy with zapping by ignoring
    the accessed bit, clearing the flood counter on page fault.



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