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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Nov 15, 2012 at 07:17:15AM +0800, Xiao Guangrong wrote:
> On 11/14/2012 10:37 PM, Marcelo Tosatti wrote:
> > 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@linux.vnet.ibm.com>
> >>>> ---
> >>>>  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.
> > 
> 
> This case is removed now, the code when e49146dce8c3dc6f44 was applied is:
> |
> |                pt = sp->spt;
> |                for (i = 0; i < PT64_ENT_PER_PAGE; ++i)
> |                        /* avoid RMW */
> |                        if (is_writable_pte(pt[i]))
> |                                update_spte(&pt[i], pt[i] & ~PT_WRITABLE_MASK);
> |        }
> 
> The real problem in this code is it would write-protect the spte even if
> it is not a last spte that caused the middle-level shadow page table was
> write-protected. So e49146dce8c3dc6f44 added this code:
> |                if (sp->role.level != PT_PAGE_TABLE_LEVEL)
> |                        continue;
> |
> was good to fix this problem.
> 
> Now, the current code is:
> |		for (i = 0; i < PT64_ENT_PER_PAGE; ++i) {
> |			if (!is_shadow_present_pte(pt[i]) ||
> |			      !is_last_spte(pt[i], sp->role.level))
> |				continue;
> |
> |			spte_write_protect(kvm, &pt[i], &flush, false);
> |		}
> It only write-protect the last spte. So, it allows large spte existent.
> (the large spte can be broken by drop_large_spte() on the page-fault path.)
> 
> > 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).
> 
> This is safe, we change these code to:
> 
> -		if (mmu_need_write_protect(vcpu, gfn, can_unsync)) {
> +		if ((level > PT_PAGE_TABLE_LEVEL &&
> +		   has_wrprotected_page(vcpu->kvm, gfn, level)) ||
> +		      mmu_need_write_protect(vcpu, gfn, can_unsync)) {
>  			pgprintk("%s: found shadow page for %llx, marking ro\n",
>  				 __func__, gfn);
>  			ret = 1;
> 
> The spte become read-only which can ensure the shadow gfn can not be changed.
> 
> Btw, the origin code allows to create readonly spte under this case if !(pte_access & WRITEABBLE)

Regarding shadow: it should be fine as long as fault path always deletes
large mappings, when shadowed pages are present in the region.

Ah, unshadowing from reexecute_instruction does not handle
large pages. I suppose that is what "simplification" refers 
to.