Re: Likely VMX Assist Issue from Ubuntu bug report

[Mats Petersson <mats@planetcatfish.com>]

At 23:02 06/09/2007, Zulauf, John wrote:
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>"fancy isolinux screen on install isos hang xen/kvm HVM guests"
>
><https://bugs.launchpad.net/ubuntu/+bug/83642>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>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.
>
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