Likely VMX Assist Issue from Ubuntu bug report

Likely VMX Assist Issue from Ubuntu bug report

[Zulauf, John]

[-- Attachment #1.1: Type: text/plain, Size: 3295 bytes --]

"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

 

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.

 


[-- Attachment #1.2: Type: text/html, Size: 7950 bytes --]

[-- Attachment #2: Type: text/plain, Size: 138 bytes --]

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

Re: Likely VMX Assist Issue from Ubuntu bug report

[Mats Petersson]

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>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.
>
>_______________________________________________
>Xen-devel mailing list
>Xen-devel@lists.xensource.com
>http://lists.xensource.com/xen-devel