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Re: [Xen-users] xen over quemu OR quemu in Xen domU on a system with HVM

To: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
Subject: Re: [Xen-users] xen over quemu OR quemu in Xen domU on a system with HVM-capable CPU
From: Igor Chubin <igor@xxxxxxx>
Date: Thu, 31 May 2007 13:55:27 +0300
Cc: Mark Williamson <mark.williamson@xxxxxxxxxxxx>, Igor Chubin <igor@xxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
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Reply-to: Igor Chubin <igor@xxxxxxx>
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> > > Ah.  I have an AMD-V box that works with FreeBSD 6 OK...  
> > Are you running on 
> > > an Intel VT-x box?
> > > 
> > 
> > Yes.
> > At this moment I use Intel VT-x box for my experiments
> > (Hewlett-Packard DL380 G5 to be more precise).
> > 
> > But I can change my hardware if I'll have good reasons for this.
> > The fact that FreeBSD runs in Xen domU's on hosts with AMD CPUs, 
> > but not run on hosts with Intel CPUs is very serious, as for me.
> > 
> > (May it be that the main reason why FreeBSD runs on one system [AMD]
> > but does not want to run on another [Intel] is not CPU, but BIOS or
> > something else?)
> HVM domains do not use the BIOS in the machine they are running on at
> all, so any BIOS difference should be completely ignored. 
> In this particular case, I'm pretty sure the reason why it doesn't work
> is that Intel's VT doesn't support real-mode guests. Instead, they
> emulate realmode in VM86 mode (so the processor is in protected 32-bit
> mode, but running 16-bit real-mode style code). This works as long as
> the instructions aren't "ring 0" instructions - when these instructions
> are seen, they trap with a GP-fault. This is then handled in the
> VMXassist code that emulates the relevant instruction. This is also
> fine. The problem occurs when a transition is made from real mode to
> protected mode and back again, where the registers (particular segment
> registers) need to be preserved - you can't do that in VM86 mode! So
> registers set in protected mode are "reset" when re-entering real-mode.
> This makes "big real mode" tricks fail [big real mode is really just
> going into protected mode, setting a segment to base=0, limit =
> 0xFFFFFFFF, and returning to real-mode - this allows real-mode code to
> access all of the first 4GB of memory without any problems, rather than
> being limited to 1MB]. Big real-mode is used by many boot-loaders. 

Thank you Mats, for this explanation.

I was aware that problem with FreeBSD in domU 
is related to "big real mode", 
but you gave many interesting details.

May I try to use GRUB to load FreeBSD kernel
and to circumvent problem with big real mode
that I face when use traditional FreeBSD /boot/loader?
What do you think about it?

> So as a conclusion, the difference here is the internal architecture of
> the processor. AMD choose the "clever way", I think. 

If I understand you right, 
there are no problems with running real-mode guests on 
AMD processors at all?

And another question:

Does anybody know something about running of such rare (for the
present) legacy operating systems, like:

* Windows NT 4
* Windows 95/98
* OS/2

and not legacy, but rare (comparing to Windows and Linux)

* OpenBSD
* Plan 9?

(I ask about running named OS as full virtual guests on a host with
HVM-capable AMD CPU)

Particular question about Plan 9.
As far as I know Plan 9 works well as paravirtualized guest
in Xen 2.
But what about Xen 3?

> --
> Mats
> _______________________________________________
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> http://lists.xensource.com/xen-users

WBR, i.m.chubin

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