From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751851Ab2KQOGa (ORCPT ); Sat, 17 Nov 2012 09:06:30 -0500 Received: from e23smtp05.au.ibm.com ([202.81.31.147]:52605 "EHLO e23smtp05.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751685Ab2KQOG2 (ORCPT ); Sat, 17 Nov 2012 09:06:28 -0500 Message-ID: <50A799DA.7050900@linux.vnet.ibm.com> Date: Sat, 17 Nov 2012 22:06:18 +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> <50A5C518.7030002@linux.vnet.ibm.com> <20121116095752.GA21401@amt.cnet> In-Reply-To: <20121116095752.GA21401@amt.cnet> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit x-cbid: 12111714-1396-0000-0000-0000022D7546 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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!