Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752594Ab2KRFFr (ORCPT ); Sun, 18 Nov 2012 00:05:47 -0500 Received: from mx1.redhat.com ([209.132.183.28]:5929 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750884Ab2KRFFq (ORCPT ); Sun, 18 Nov 2012 00:05:46 -0500 Date: Sun, 18 Nov 2012 01:00:41 -0200 From: Marcelo Tosatti To: Xiao Guangrong Cc: Avi Kivity , LKML , KVM Subject: Re: [PATCH] KVM: MMU: lazily drop large spte Message-ID: <20121118030040.GA5334@amt.cnet> References: <20121112231032.GB5798@amt.cnet> <50A20428.1030004@linux.vnet.ibm.com> <20121114143747.GA7054@amt.cnet> <50A4267B.1030902@linux.vnet.ibm.com> <20121116030222.GA21822@amt.cnet> <50A5B560.2070604@linux.vnet.ibm.com> <20121116035655.GA27414@amt.cnet> <50A5C518.7030002@linux.vnet.ibm.com> <20121116095752.GA21401@amt.cnet> <50A799DA.7050900@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <50A799DA.7050900@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 7258 Lines: 166 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 > >>>>>>>>>> --- > >>>>>>>>>> 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). -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/