Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751556Ab2KPJ6D (ORCPT ); Fri, 16 Nov 2012 04:58:03 -0500 Received: from mx1.redhat.com ([209.132.183.28]:52872 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751391Ab2KPJ6B (ORCPT ); Fri, 16 Nov 2012 04:58:01 -0500 Date: Fri, 16 Nov 2012 07:57:52 -0200 From: Marcelo Tosatti To: Xiao Guangrong Cc: Avi Kivity , LKML , KVM Subject: Re: [PATCH] KVM: MMU: lazily drop large spte Message-ID: <20121116095752.GA21401@amt.cnet> References: <50978DFE.1000005@linux.vnet.ibm.com> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <50A5C518.7030002@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: 6581 Lines: 156 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. -- 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/