From mboxrd@z Thu Jan 1 00:00:00 1970 From: Xiao Guangrong Subject: Re: [PATCH 5/5] KVM: MMU: fast invalid all mmio sptes Date: Mon, 18 Mar 2013 21:25:10 +0800 Message-ID: <514715B6.3080507@linux.vnet.ibm.com> References: <51433D98.4050605@linux.vnet.ibm.com> <51433E71.2070107@linux.vnet.ibm.com> <20130317150239.GM11223@redhat.com> <5146CB92.7060203@linux.vnet.ibm.com> <20130318091303.GA1276@redhat.com> <514708A9.8040009@linux.vnet.ibm.com> <20130318124616.GH4020@redhat.com> <51471217.7070801@linux.vnet.ibm.com> <20130318131907.GK4020@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , LKML , KVM To: Gleb Natapov Return-path: Received: from e28smtp08.in.ibm.com ([122.248.162.8]:48774 "EHLO e28smtp08.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750899Ab3CRNZT (ORCPT ); Mon, 18 Mar 2013 09:25:19 -0400 Received: from /spool/local by e28smtp08.in.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 18 Mar 2013 18:50:46 +0530 In-Reply-To: <20130318131907.GK4020@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 03/18/2013 09:19 PM, Gleb Natapov wrote: > On Mon, Mar 18, 2013 at 09:09:43PM +0800, Xiao Guangrong wrote: >> On 03/18/2013 08:46 PM, Gleb Natapov wrote: >>> On Mon, Mar 18, 2013 at 08:29:29PM +0800, Xiao Guangrong wrote: >>>> On 03/18/2013 05:13 PM, Gleb Natapov wrote: >>>>> On Mon, Mar 18, 2013 at 04:08:50PM +0800, Xiao Guangrong wrote: >>>>>> On 03/17/2013 11:02 PM, Gleb Natapov wrote: >>>>>>> On Fri, Mar 15, 2013 at 11:29:53PM +0800, Xiao Guangrong wrote: >>>>>>>> This patch tries to introduce a very simple and scale way to invalid all >>>>>>>> mmio sptes - it need not walk any shadow pages and hold mmu-lock >>>>>>>> >>>>>>>> KVM maintains a global mmio invalid generation-number which is stored in >>>>>>>> kvm->arch.mmio_invalid_gen and every mmio spte stores the current global >>>>>>>> generation-number into his available bits when it is created >>>>>>>> >>>>>>>> When KVM need zap all mmio sptes, it just simply increase the global >>>>>>>> generation-number. When guests do mmio access, KVM intercepts a MMIO #PF >>>>>>>> then it walks the shadow page table and get the mmio spte. If the >>>>>>>> generation-number on the spte does not equal the global generation-number, >>>>>>>> it will go to the normal #PF handler to update the mmio spte >>>>>>>> >>>>>>>> Since 19 bits are used to store generation-number on mmio spte, the >>>>>>>> generation-number can be round after 33554432 times. It is large enough >>>>>>>> for nearly all most cases, but making the code be more strong, we zap all >>>>>>>> shadow pages when the number is round >>>>>>>> >>>>>>> Very nice idea, but why drop Takuya patches instead of using >>>>>>> kvm_mmu_zap_mmio_sptes() when generation number overflows. >>>>>> >>>>>> I am not sure whether it is still needed. Requesting to zap all mmio sptes for >>>>>> more than 500000 times is really really rare, it nearly does not happen. >>>>>> (By the way, 33554432 is wrong in the changelog, i just copy that for my origin >>>>>> implantation.) And, after my patch optimizing zapping all shadow pages, >>>>>> zap-all-sps should not be a problem anymore since it does not take too much lock >>>>>> time. >>>>>> >>>>>> Your idea? >>>>>> >>>>> I expect 500000 to become less since I already had plans to store some >>>> >>>> Interesting, just curious, what are the plans? ;) >>>> >>> Currently we uses pio to signal that work is pending to virtio devices. The >>> requirement is that signaling should be fast and PIO is fast since there >>> is not need to emulate instruction. PCIE though is not really designed >>> with PIO in mind, so we will have to use MMIO to do signaling. To avoid >>> instruction emulation I thought about making guest access these devices using >>> predefined variety of MOV instruction so that emulation can be skipped. >>> The idea is to mark mmio spte to know that emulation is not needed. >> >> How to know page-fault is caused by the predefined instruction? >> > Only predefined phys address rages will be accessed that way. If page > fault is in a range we assume the knows instruction is used. That means the access can be identified by the gfn, why need cache other things into mmio spte?