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 D8126C432C2 for ; Thu, 26 Sep 2019 09:00:16 +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 B127F222C3 for ; Thu, 26 Sep 2019 09:00:16 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B127F222C3 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]:60978 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iDPdH-0003C6-SU for qemu-devel@archiver.kernel.org; Thu, 26 Sep 2019 05:00:15 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:51386) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1iDPcO-000233-FY for qemu-devel@nongnu.org; Thu, 26 Sep 2019 04:59:21 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1iDPcM-0007gn-ET for qemu-devel@nongnu.org; Thu, 26 Sep 2019 04:59:20 -0400 Received: from mx1.redhat.com ([209.132.183.28]:59502) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1iDPcM-0007eE-8A for qemu-devel@nongnu.org; Thu, 26 Sep 2019 04:59:18 -0400 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 6E310796E0; Thu, 26 Sep 2019 08:59:17 +0000 (UTC) Received: from maximlenovopc.usersys.redhat.com (unknown [10.35.206.33]) by smtp.corp.redhat.com (Postfix) with ESMTP id 55C975C21F; Thu, 26 Sep 2019 08:59:16 +0000 (UTC) Message-ID: <3d3f3a0e6e796260348c66e69e859e1901501ee8.camel@redhat.com> Subject: Re: Questions about the real mode in kvm/qemu From: Maxim Levitsky To: Li Qiang Date: Thu, 26 Sep 2019 11:59:15 +0300 In-Reply-To: References: <644968ffb11c11fd580e96c1e67932501a633fe4.camel@redhat.com> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Thu, 26 Sep 2019 08:59:17 +0000 (UTC) Content-Transfer-Encoding: quoted-printable 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: Paolo Bonzini , Qemu Developers Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, 2019-09-26 at 16:52 +0800, Li Qiang wrote: >=20 >=20 > Maxim Levitsky =E4=BA=8E2019=E5=B9=B49=E6=9C=8826= =E6=97=A5=E5=91=A8=E5=9B=9B =E4=B8=8B=E5=8D=884:31=E5=86=99=E9=81=93=EF=BC= =9A > > On Thu, 2019-09-26 at 15:52 +0800, Li Qiang wrote: > > > Hi Paolo and all, > > >=20 > > > There are some question about the emulation for real mode in kvm/qe= mu. For all the=20 > > > question I suppose the 'unstrict guest' is not enabled.=20 > > >=20 > > > 1. how the protected mode CPU emulate the real mode? It seems it us= es vm86, however, vm86 is not available in x86_64 CPU? So what's the 'to_= vmx(vcpu)->rmode.vm86_active' here vm86 means? > > >=20 > >=20 >=20 > Hi Maxim, >=20 > Thanks for your kind reply. >=20 > =20 > > As far as I know it, modern intel's cpus support so called unrestrict= ed guest mode, which allows guest to be basically in any mode, >=20 > Right, but I also want to know the secret when the 'unstrict guest' is = disabled. So I suppose the 'unstrict guest' is not enabled for these que= stions. > =20 > > as long as EPT paging is used (that is guest can be in real mode with > > no paging, but EPT has to be enabled). > > The 'vm86_active' is probably lefover support for cpus that don't sup= port EPT and/or the unrestricted guest mode, > > where KVM tried to use the good old vm86 mode to > > for real mode virtualization. > >=20 > >=20 > > > 2. Does the guest's real mode code run directly in native CPU? It s= eems 'vmx->emulation_required' is also be false, it the vmx_vcpu_run will= do a switch to guest. > >=20 > > Same as above > >=20 > > >=20 > > > 3. How the EPT work in guest real mode? The EPT is for GVA->GPA->HP= A, however there is no GVA, seems the identity mapping does something. Bu= t there also some confusion for me. For example the > > real > > > mode uses CS*4 + IP to address the code. Who does this calculation= ? In the kernel emulator?=20 > >=20 > > EPT sits underneath the guest's paging mode, which in case of real mo= de is 1:1 mapping. >=20 > It seems when the 'unstrict guest' is enabled, there is no identity map= ping table. If you mean to ask if there is a way to let guest access use no paging at= all, that is access host physical addresses directly, then indeed there is no way, since regular non 'unrestricted guest' mode = required both protected mode and paging, and 'unrestricted guest' require= s EPT. Academically speaking it is of course possible to create paging tables th= at are 1:1... Best regards, Maxim Levitsky