From mboxrd@z Thu Jan 1 00:00:00 1970 From: Xiao Guangrong Subject: Re: [PATCH 11/11] KVM: MMU: improve write flooding detected Date: Thu, 25 Aug 2011 15:40:56 +0800 Message-ID: <4E55FC88.4040300@cn.fujitsu.com> References: <4E4A10E8.5090705@cn.fujitsu.com> <4E4A1257.5080204@cn.fujitsu.com> <20110823080024.GA2297@amt.cnet> <4E53872B.3070407@cn.fujitsu.com> <20110823123818.GB4261@amt.cnet> <4E53D620.9030304@cn.fujitsu.com> <20110823190939.GA10220@amt.cnet> <4E540AB4.6060905@cn.fujitsu.com> <20110824200540.GA19804@amt.cnet> <20110825020424.GA2542@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: Received: from cn.fujitsu.com ([222.73.24.84]:63171 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1750898Ab1HYHit (ORCPT ); Thu, 25 Aug 2011 03:38:49 -0400 In-Reply-To: <20110825020424.GA2542@amt.cnet> Sender: kvm-owner@vger.kernel.org List-ID: On 08/25/2011 10:04 AM, Marcelo Tosatti wrote: >>> Yes, in this case, the sp is not zapped, but it is hardly to know the gfn >>> is not used as gpte just depends on writing, for example, the guest can >>> change the mapping address or the status bit, and so on...The sp can be >>> zapped if the guest write it again(on the same address), i think it is >>> acceptable, anymore, it is just the speculative way to zap the unused >>> shadow page...your opinion? >> >> It could increase the flood count independently of the accessed bit of >> the spte being updated, zapping after 3 attempts as it is now. >> >> But additionally reset the flood count if the gpte appears to be valid >> (points to an existant gfn if the present bit is set, or if its zeroed). > > Well not zero, as thats a common pattern for non ptes. > Hi Marcelo, Maybe it is not good i think, for some reasons: - checking gfn valid which it is pointed by gpte is high overload, it needs to call gfn_to_hva to walk memslots, especially. kvm_mmu_pte_write is called very frequently on shadow mmu. - MMIO gfn is not an existent gfn, but it is valid pointed by gpte - we can check the reserved bits in the gpte to check whether it is valid a gpte, but for some paging modes, all bits are valid.(for example, non-PAE mode) - it can not work if the gfn has multiple shadow pages, for example: if the gfn was used as PDE, later it is used as PTE, then we have two shadow pages: sp1.level = 2, sp2.level = 1, sp1 can not be zapped even even though it is not used anymore. - sometime, we need to zap the shadow page even though the gpte is written validly: if the gpte is written frequently but infrequently accessed, we do better zap the shadow page to let it is writable(write it directly without #PF) and map it when it is accessed, one example is from Avi, the guest OS may update many gptes at one time after one page fault.