Re: [PATCH] KVM: MMU: lazily drop large spte

[Marcelo Tosatti]

On Tue, Nov 13, 2012 at 04:26:16PM +0800, Xiao Guangrong wrote:
> Hi Marcelo,
> 
> On 11/13/2012 07:10 AM, Marcelo Tosatti wrote:
> > On Mon, Nov 05, 2012 at 05:59:26PM +0800, Xiao Guangrong wrote:
> >> Do not drop large spte until it can be insteaded by small pages so that
> >> the guest can happliy read memory through it
> >>
> >> The idea is from Avi:
> >> | As I mentioned before, write-protecting a large spte is a good idea,
> >> | since it moves some work from protect-time to fault-time, so it reduces
> >> | jitter.  This removes the need for the return value.
> >>
> >> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
> >> ---
> >>  arch/x86/kvm/mmu.c |   34 +++++++++-------------------------
> >>  1 files changed, 9 insertions(+), 25 deletions(-)
> > 
> > Its likely that other 4k pages are mapped read-write in the 2mb range 
> > covered by a read-only 2mb map. Therefore its not entirely useful to
> > map read-only. 
> > 
> 
> It needs a page fault to install a pte even if it is the read access.
> After the change, the page fault can be avoided.
> 
> > Can you measure an improvement with this change?
> 
> I have a test case to measure the read time which has been attached.
> It maps 4k pages at first (dirt-loggged), then switch to large sptes
> (stop dirt-logging), at the last, measure the read access time after write
> protect sptes.
> 
> Before: 23314111 ns	After: 11404197 ns

Ok, i'm concerned about cases similar to e49146dce8c3dc6f44 (with shadow),
that is:

- large page must be destroyed when write protecting due to 
shadowed page.
- with shadow, it does not make sense to write protect 
large sptes as mentioned earlier.

So i wonder why is this part from your patch

-               if (level > PT_PAGE_TABLE_LEVEL &&
-                   has_wrprotected_page(vcpu->kvm, gfn, level)) {
-                       ret = 1;
-                       drop_spte(vcpu->kvm, sptep);
-                       goto done;
-               }

necessary (assuming EPT is in use).

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