From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 31C4FC432C2 for ; Thu, 26 Sep 2019 10:05:26 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 078B0217F4 for ; Thu, 26 Sep 2019 10:05:26 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 078B0217F4 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:33206 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iDQeK-0000D0-Vn for qemu-devel@archiver.kernel.org; Thu, 26 Sep 2019 06:05:25 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:33908) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iDQcu-0007Zm-9g for qemu-devel@nongnu.org; Thu, 26 Sep 2019 06:03:57 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iDQct-0006kM-59 for qemu-devel@nongnu.org; Thu, 26 Sep 2019 06:03:56 -0400 Received: from mx1.redhat.com ([209.132.183.28]:41434) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iDQcs-0006kD-VU for qemu-devel@nongnu.org; Thu, 26 Sep 2019 06:03:55 -0400 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 26CAFC00A167; Thu, 26 Sep 2019 10:03:54 +0000 (UTC) Received: from maximlenovopc.usersys.redhat.com (unknown [10.35.206.33]) by smtp.corp.redhat.com (Postfix) with ESMTP id DD5CF60BE0; Thu, 26 Sep 2019 10:03:52 +0000 (UTC) Message-ID: Subject: Re: Questions about the real mode in kvm/qemu From: Maxim Levitsky To: Paolo Bonzini , Li Qiang Date: Thu, 26 Sep 2019 13:03:51 +0300 In-Reply-To: <4a8d36ed-4a3b-fd12-303d-fc1ee50b2980@redhat.com> References: <644968ffb11c11fd580e96c1e67932501a633fe4.camel@redhat.com> <3d3f3a0e6e796260348c66e69e859e1901501ee8.camel@redhat.com> <23789310-35fb-8c93-44f4-532bcd34007d@redhat.com> <7c019f3a5236daaa79e67467f64cde212ad05f35.camel@redhat.com> <4ed0f9ca-6cd1-fd8e-9abd-4098f85c7f9d@redhat.com> <4a8d36ed-4a3b-fd12-303d-fc1ee50b2980@redhat.com> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.32]); Thu, 26 Sep 2019 10:03:54 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 209.132.183.28 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Qemu Developers , Avi Kivity Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, 2019-09-26 at 12:00 +0200, Paolo Bonzini wrote: > On 26/09/19 11:41, Maxim Levitsky wrote: > > > Thinking more about it, I suppose that saves memory (the same EPT page > > > tables can now be used independent of guest CR0.PG), at the cost of > > > making TLB misses a little slower. > > > > Don't really understand what you mean. > > Isn't this always the case that EPT and guest paging > > are independent (at least when no nesting is involved)? > > There are two possibilities: > > 1) emulate CR0.PG=0 with EPT + identity page > > - advantage: the EPT pages will be reused once the guest sets CR0.PG=1 > > - disadvantage: TLB misses have to walk two levels of page tables > > 2) emulate CR0.PG=0 with EPT disabled. Similar to ept=0, CR3 will point > to PAE page tables that do the HVA->GPA transition. > > - advantage: faster TLB misses > > - disadvantage: need to build separate page tables for CR0.PG=1 (EPT > format) and CR0.PG=0 (PAE format), need to "waste" 4k of GPA space for > the identity map Thanks for the explanation! Best regards, Maxim Levitsky