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

[Marcelo Tosatti]

On Sat, Nov 17, 2012 at 10:06:18PM +0800, Xiao Guangrong wrote:
> On 11/16/2012 05:57 PM, Marcelo Tosatti wrote:
> > On Fri, Nov 16, 2012 at 12:46:16PM +0800, Xiao Guangrong wrote:
> >> On 11/16/2012 11:56 AM, Marcelo Tosatti wrote:
> >>> 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. refault, then goto 2 (as part of write to gfn 4)
> >>
> >> Hmm, it is not true. :)
> >>
> >> The large spte can become writable since 'no pages adhadoes' (that means
> >> has_wrprotected_page() can return 0 for this case). No?
> > 
> > What if gfn 4 is a pagetable part of the pagedirectory chain used to 
> > map gfn 4?  See corrected step 7 above.
> 
> Ah, this is a real bug, and unfortunately, it exists in current
> code. I will make a separate patchset to fix it. Thank you, Marcelo!

Is it? Hum..

Anyway, it would be great if you can write a testcase (should be similar
in size to rmap_chain).

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