Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751453Ab2KPEq0 (ORCPT ); Thu, 15 Nov 2012 23:46:26 -0500 Received: from e28smtp01.in.ibm.com ([122.248.162.1]:60717 "EHLO e28smtp01.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750971Ab2KPEqY (ORCPT ); Thu, 15 Nov 2012 23:46:24 -0500 Message-ID: <50A5C518.7030002@linux.vnet.ibm.com> Date: Fri, 16 Nov 2012 12:46:16 +0800 From: Xiao Guangrong User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120911 Thunderbird/15.0.1 MIME-Version: 1.0 To: Marcelo Tosatti CC: Avi Kivity , LKML , KVM Subject: Re: [PATCH] KVM: MMU: lazily drop large spte 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> In-Reply-To: <20121116035655.GA27414@amt.cnet> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit x-cbid: 12111604-4790-0000-0000-0000059651CD Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6062 Lines: 153 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 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). 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