Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751431Ab2KPD5C (ORCPT ); Thu, 15 Nov 2012 22:57:02 -0500 Received: from mx1.redhat.com ([209.132.183.28]:61804 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750960Ab2KPD5A (ORCPT ); Thu, 15 Nov 2012 22:57:00 -0500 Date: Fri, 16 Nov 2012 01:56:55 -0200 From: Marcelo Tosatti To: Xiao Guangrong Cc: Avi Kivity , LKML , KVM Subject: Re: [PATCH] KVM: MMU: lazily drop large spte Message-ID: <20121116035655.GA27414@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <50A5B560.2070604@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: 6059 Lines: 151 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. 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. > ) -- 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/