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

On 08/24/2011 03:09 AM, Marcelo Tosatti wrote:

On Wed, Aug 24, 2011 at 12:32:32AM +0800, Xiao Guangrong wrote:

quoted

On 08/23/2011 08:38 PM, Marcelo Tosatti wrote:

quoted

quoted

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.

quoted

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.

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.

quoted

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.