Re: A bunch of questions about getting a working NetBSD/xen system

[Steven Bellovin <smb%cs.columbia.edu@localhost>]

On Feb 2, 2010, at 6:16 PM, Louis Guillaume wrote:

> Hi!
> 
> I've been struggling for a month or two now with a xen install and have a few 
> general questions that I'm hoping will clear up my foggy understanding of how 
> things are supposed to work:
> 
> 0. The goal: To have a NetBSD/xen dom0 machine on which to host guest
>   machines. Ideally this should:
> 
>     o run on netbsd-x-stable whatever that is at the time.
>     o run unmodified oses; i.e. HVM support.
> 
> 1. What I have:
> 
>     o Dual 3Ghz 64-bit Xeons.
>     o 4G Ram.
>     o Tyan S5360 Mb.
> 
>   From looking at the CPU specs it would appear that they have the
>   Intel VT technology needed for HVM support.
> 
>   It does not appear that my BIOS supports this though. If I use
>   "cpuctl identify" I do not see the "VT" instruction listed, and
>   nowhere in my dmesg output do I have an indication that it's
>   turned off in the Bios as some sources (NetBSD wiki's xen page)
>   suggest.
> 
>   So - first question: Will it work? Or am I stuck with non-HVM and
>   only modified kernels?
> 
>   Is there a way to make the system HVM-aware when the BIOS
>   apparently isn't?

You say you're running a -current kernel.  What version is your cpuctl command 
from?  And what flag are you looking for?  I think it's supposed to say "vmx" 
(on Intel), rather than "vt".
> 
> 
> 2. I've been running a -current kernel as it's been suggested that
>   only -current kernels will work. But then I've seen somewhere
>   that netbsd-5 should work fine.
> 
>   Can I use netbsd-5 instead of -current for DOM0/PAE yet?
> 
Yes -- I'm doing it.  But I haven't tried i386 guests or HVM guests.

> 6. What's the deal with SMP on xen? I see the following in dmesg:
> 
>     cpu0 at mainbus0 apid 0: (boot processor)
>     cpu1 at mainbus0 apid 6: (application processor)
>     cpu2 at mainbus0 apid 1: (application processor)
>     cpu3 at mainbus0 apid 7: (application processor)
> 
>   and things like "top" only show one processor active. Last time I
>   think I saw this separation of "boot" vs. "application" processor
>   was in netbsd-2 or -3, I think.
> 
>   What should I expect to see here in DOM0?

The dom0 only runs on cpu0, I believe, but different guests can run on 
different CPUs simultaneously.  In other words, the dom0 is in some weak sense 
SMP, but only weakly.  That said, for many purposes -- giving separate VMs a 
full CPU -- it works fine, but not for an MP guest.

I don't know what happens if you try to do, say, system builds on the DOM0 -- 
can it use multiple processors?  I suspect not, but I'm of the minimalist DOM0 
religion.
> 
>   Can I run SMP guests?

No.

I've thought about running a Linux DOM0 and NetBSD DOMUs, but mainstream Linux 
seems to be abandoning Xen, so I'd have to run a back release.

                --Steve Bellovin, http://www.cs.columbia.edu/~smb