Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758199Ab0GEItn (ORCPT ); Mon, 5 Jul 2010 04:49:43 -0400 Received: from cn.fujitsu.com ([222.73.24.84]:51427 "EHLO song.cn.fujitsu.com" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1753800Ab0GEItl (ORCPT ); Mon, 5 Jul 2010 04:49:41 -0400 Message-ID: <4C319BBB.5020408@cn.fujitsu.com> Date: Mon, 05 Jul 2010 16:45:47 +0800 From: Xiao Guangrong User-Agent: Thunderbird 2.0.0.24 (Windows/20100228) MIME-Version: 1.0 To: Avi Kivity CC: Marcelo Tosatti , LKML , KVM list Subject: Re: [PATCH v4 5/6] KVM: MMU: combine guest pte read between walk and pte prefetch References: <4C2C9DC0.8050607@cn.fujitsu.com> <4C2C9E6C.2040803@cn.fujitsu.com> <20100702170303.GC25969@amt.cnet> <4C2F117C.2000006@cn.fujitsu.com> <4C2F2835.5060508@redhat.com> <4C2F2A0C.90704@cn.fujitsu.com> <4C2F2C5B.9020503@redhat.com> <4C2F2DBB.50904@cn.fujitsu.com> <4C2F30BD.7050702@redhat.com> <4C2F31D9.5010104@redhat.com> <4C2F3527.3020307@cn.fujitsu.com> <4C309B23.9060808@redhat.com> <4C3148FF.3030209@cn.fujitsu.com> <4C319699.9000104@redhat.com> In-Reply-To: <4C319699.9000104@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2168 Lines: 61 Avi Kivity wrote: >> Looks into the code more carefully, maybe this code is wrong: >> >> >> if (!direct) { >> r = kvm_read_guest_atomic(vcpu->kvm, >> - gw->pte_gpa[level - 2], >> + gw->pte_gpa[level - 1], >> &curr_pte, sizeof(curr_pte)); >> - if (r || curr_pte != gw->ptes[level - 2]) { >> + if (r || curr_pte != gw->ptes[level - 1]) { >> kvm_mmu_put_page(sp, sptep); >> kvm_release_pfn_clean(pfn); >> sptep = NULL; >> >> It should check the 'level' mapping not 'level - 1', in the later >> description >> i'll explain it. >> > > Right, this fixes the check for the top level, but it removes a check > from the bottom level. > We no need check the bottom level if guest not modify the bottom level, if guest modify it, the bottom level is no-present, it also can broke Point A's judgment and be checked by 'Point C' > We need to move this to the top of the loop so we check all levels. I > guess this is why you needed to add a new check point. But since we > loop at least glevels times, we don't need two check points. > > > Ok. So moving the check to before point A, and s/level - 2/level - 1/ > should work, yes? > > Should be slightly simpler since we don't need to kvm_mmu_put_page(sp, > sptep) any more. Yeah, it can work, but check all levels is really unnecessary, if guest not modify the level, the check can be avoid. This is why i choose two check-point, one is behind Point A's judgment, this point checks the level which modified by guest, and another point is at mapping last level point, this check is alway need. > > Thanks for the detailed explanation. > It's really my pleasure :-) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/