From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mats Petersson Subject: Re: Likely VMX Assist Issue from Ubuntu bug report Date: Thu, 06 Sep 2007 23:13:37 +0100 Message-ID: <46e07b6e.1185300a.4139.05c4@mx.google.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; format=flowed Return-path: In-Reply-To: References: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: "Zulauf, John" , xen-devel@lists.xensource.com List-Id: xen-devel@lists.xenproject.org At 23:02 06/09/2007, Zulauf, John wrote: >Content-class: urn:content-classes:message >Content-Type: multipart/alternative; > boundary="----_=_NextPart_001_01C7F0D1.A664E607" > >"fancy isolinux screen on install isos hang xen/kvm HVM guests" > >https://bugs.launchpad.net/ubuntu/+bug/83642 > >Bug is in state "confirmed" and while the concensus on the bug >thread is "we probably shouldn't be doing fancy things in protected >mode", this seems like a fairly important distro to be borked for >3.2 (or 3.1.1 whatever). > >I've verified this using the i386 Ubuntu tip: >http://cdimage.ubuntu.com/releases/gutsy/tribe-5/gutsy-desktop-i386.iso As far as I know, this is still the case. And the reason is, as stated in the bug-report, that the Intel people decided to not allow real-mode code to be virtualized. This is not a problem in itself, because VM8086 mode can cover most of the code, with a bit of "VMXAssist" to fix up privileged instructions not allows in VM8086 mode. However, there is a problem, and that is that the state of segment registers isn't maintained when switching from protected mode back to real-mode. This is often used by boot-loading code to load data into memory that is normally not reachable from real-mode (that is, memory above 1M). -- Mats > >FYI > >John Zulauf > >Quoting the Ubuntu bug: > >I'm working with ubuntu on Xen right now, and this is a very bad >problem. Both the server and desktop install CD's, for both dapper >and edgy, use fancy full screen (vbe based?) isolinux install >screens. Could we please do away with these screens on the server or >alternate install CD? These fancy screens are causing fully >virtualized guests that try to boot from these CD's to crash before >the boot screen is even drawn. This is true for both Xen and KVM. > >To clarify, this does not cause the -host- to crash, just the guest, >but you get a blank screen and wonder what happened. > >Background (mostly speculation, but I think it's right): >Intel designed VT in a strange way, such that only protected mode >instructions are allowed o be virtualized. Anything done in real >mode must be emulated...every instruction. The Xen and KVM folks >have emulated enough to get things working, but have not handled >every instruction, including apparently some of the fancy (VBE?) >graphics isolinux on the ubuntu boot cd's is using. (FreeBSD's btx >loader also has problems) > >For reference, AMD's SVM analogue does, in fact, virtualize real >mode instructions on the processor, and I'm able to boot all install >CD's just fine on an AMD machine. This is an Intel VT .... 'quirk'. > >The mini.iso network install CD loads just fine and works, and I can >install over an emulated network. And that CD includes a splash >screen w/ isolinux, so it's not all VBE operations that cause >problems. You should still be able to have a splash screen, but more >then that might cause the problems. > >I've filed a bug against Xen about this, and I just tried KVM and >ran into the same issue, so I'm assuming it's th same problem. But >until the Xen folks figure out what instructions need to be >emulated, and take enough time to care, or Intel comes out with an >updated VT implementation that does what AMD's does, we can't just >boot up a ubuntu install CD on virtual machines and have it work. > >My requested solution is that isolinux use just the standard >(preferably text) menu and options on the server and/or alternate >install CD. Having one full, officially supported install CD that >doesn't crash Xen/KVM when trying to install a ubuntu would go a >long way towards helping those of us trying to use ubuntu in a >virtual environment out. Besides, while I really appreciate all the >fancy boot options for a desktop machine, but I would prefer a plain >text option for servers anyway. I do, however, see why some would >like the opposite. > >Would it be possible to get the feisty 'server' install CD to forgo >the fancy bootup options? > >NOTE: I think I understand what's going on, but if I'm missing >something obvious, please tell me. > >_______________________________________________ >Xen-devel mailing list >Xen-devel@lists.xensource.com >http://lists.xensource.com/xen-devel