Re: [Xen-devel] Likely VMX Assist Issue from Ubuntu bug report
At 23:02 06/09/2007, Zulauf, John wrote:
"fancy isolinux screen on install isos hang xen/kvm HVM guests"
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:
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).
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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