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

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: Igor Chubin [mailto:igor@xxxxxxx] 
> Sent: 31 May 2007 11:55
> To: Petersson, Mats
> Cc: Igor Chubin; Mark Williamson; xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] xen over quemu OR quemu in Xen domU 
> on a system with HVM-capable CPU
> 
> ...
> > > > 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.
> 
> Question.
> 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

I tried this just a few weeks ago. Worked for the limited amount of
testing I gave it. 

> * Windows 95/98

I've got a disk sitting around, but I've not tested it. I spent a few
minutes now trying to get it to run, but it seems to not do so - not
sure why. 

> * OS/2
I haven't tested myself, but list-member Trolle Selander is using this
combination for commercial purposes. 

> 
> and not legacy, but rare (comparing to Windows and Linux)
> 
> * OpenBSD
> * MINIX
> and 
> * Plan 9?

I haven't tried any of them. The problem(s) with using "unknown" OS's is
that they may perform operations that aren't supported by the HVM part
of Xen - it's usually not THAT hard to fix, but it can be sometimes...
:-(

--
Mats
> 
> (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
> > 
> > 
> > 
> > 
> > _______________________________________________
> > Xen-users mailing list
> > Xen-users@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-users
> 
> -- 
> WBR, i.m.chubin
> 
> 
> 
> 



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