From mboxrd@z Thu Jan 1 00:00:00 1970 From: Xiao Guangrong Subject: Re: [PATCH v6 5/9] KVM: MMU: introduce SPTE_MMU_WRITEABLE bit Date: Wed, 13 Jun 2012 11:11:19 +0800 Message-ID: <4FD804D7.5000802@linux.vnet.ibm.com> References: <4FC470C7.5040700@linux.vnet.ibm.com> <4FC4716A.8030304@linux.vnet.ibm.com> <20120611233212.GA29573@amt.cnet> <4FD6A833.6060206@linux.vnet.ibm.com> <20120613020103.GB3119@amt.cnet> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Cc: Avi Kivity , LKML , KVM To: Marcelo Tosatti Return-path: In-Reply-To: <20120613020103.GB3119@amt.cnet> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 06/13/2012 10:01 AM, Marcelo Tosatti wrote: > On Tue, Jun 12, 2012 at 10:23:47AM +0800, Xiao Guangrong wrote: >> On 06/12/2012 07:32 AM, Marcelo Tosatti wrote: >> >>> On Tue, May 29, 2012 at 02:49:14PM +0800, Xiao Guangrong wrote: >>>> This bit indicates whether the spte can be writable on MMU, that means >>>> the corresponding gpte is writable and the corresponding gfn is not >>>> protected by shadow page protection >>> >>> Why is this still necessary, now that only sptes of direct shadow pages >>> are updated locklessly? >>> >> >> >> Yes, but it is still needed, for nested npt/ept, we need protect >> the nested page tables. > > Sure, but shadowed L1 nested pagetables are not direct shadow pages. > > They are shadows of L1 nested pagetables. > > Checking sp->direct should be enough (instead of the flags). > Hi Marcelo, I think it is not enough, for example: - In host (L0), spte1 is pointing to gfn1, spte1 is a direct spte. - in L1, L1 guest is using gfn1 in L1's ept page table for L2 guest, so, in host, we have a indirect spte (named spte2) whose sp->gfn = gfn1. Since spte2 is a indirect spte, we need protect it, so, we walk all gfn1's rmaps, spte1 will be found, then, we write-protect on spte1 to track L1 modifying gfn1. In this case, spte1 is direct but need write-protect. :)