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

[Marcelo Tosatti]

On Fri, Nov 16, 2012 at 11:39:12AM +0800, Xiao Guangrong wrote:
> On 11/16/2012 11:02 AM, Marcelo Tosatti wrote:
> > 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@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.
> >>>
> >>
> >> 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.
> 
> For hard mmu is also safe, in this patch i added these code:
> 
> @@ -2635,6 +2617,8 @@ static int __direct_map(struct kvm_vcpu *vcpu, gpa_t v, int write,
>  			break;
>  		}
> 
> +		drop_large_spte(vcpu, iterator.sptep);
> +
> 
> It can delete large mappings like soft mmu does.
> 
> Anything i missed?
> 
> > 
> > Ah, unshadowing from reexecute_instruction does not handle
> > large pages. I suppose that is what "simplification" refers 
> > to.
> 
> reexecute_instruction did not directly handle last spte, it just
> removes all shadow pages, then let cpu retry the instruction, the
> page can become writable when encounter #PF again, large spte is fine
> under this case.

While searching for a given "gpa", you don't find large gfn which is
mapping it, right? (that is, searching for gfn 4 fails to find large
read-only "gfn 0"). Unshadowing gfn 4 will keep large read-only mapping
present.

1. large read-write spte to gfn 0
2. shadow gfn 4
3. write-protect large spte pointing to gfn 0
4. write to gfn 4
5. instruction emulation fails
5. unshadow gfn 4
6. refault, do not drop large spte because no pages shadowed
7. goto 4

> (Out of this thread: I notice reexecute_instruction allows to retry
>  instruct only if tdp_enabled == 0, but on nested npt, it also has
>  page write-protected by shadow pages. Maybe we need to improve this
>  restriction.
> )
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