Re: [Xen-devel] TCP state between domU and client

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > Second question ,Can I run a Xen hypervisor inside a domU,
> > making this domU as the host and subsequent domUs  as guests
> > running over this guest domU ? Just asking this out of curiosity.
>
> Yes, no, maybe. You can (so I've been told, I haven't tried) run
> Xenified Linux on top of a HVM DomU. Obviously, any guest on top of that
> would not be able to use the HVM feature, as that is a "single-layer".

I do most of my testing of development versions of Xen within an HVM domain 
running on xen-unstable.  My reasons for this are as follows:

1) I only need paravirtual domains for my work, and Xen running in an HVM 
domain supports them just fine.
2) My real physical machine doesn't have a serial console or a remote power 
switch.  Using xm console and xm reboot, my HVM domain does.
3) I can have several different HVM domains testing different Xen versions 
configured at once.
4) The host machine can be maintained at a known-good version of Xen.

Tricks with suspend / resume could be used to make working with these even 
more efficient; I've not tried that yet.

> But for para-virtual domains, you can't run the hypervisor on top of
> itself, because the hypervisor needs Ring0 (the most privileged
> protection level). Any subsequent load would fail to control this level,
> so it's not possible to run the hypervisor at this level.

Yep.  Without HVM support, you're limited to using something like Qemu / 
VMware...  I'm working on being able to rsync my Xen development domain over 
to my laptop (no HVM) and run it under Qemu when away from my dedicated test 
box.

The only limitations I've found with my setup are:
1) It's slower than native.
2) Compiling PV-on-HVM drivers for the guest's dom0 gets a bit confused and 
doesn't work (I just use ioemu devices instead).
3) Xenoprof doesn't work for a Xen running within an HVM domain - I'm guessing 
NMIs aren't being delivered or something like that, but it's not a big 
problem for most of my work.

Cheers,
Mark

> It is also pretty pointless (aside from the fun and games you can have
> with it, and I guess it's proving a point, in the same way as compiling
> the source for the compiler with itself proves that the compiler is
> working pretty well [This can also be more useful, as the compiler is
> perhaps able to do optimization that isn't available in the previous
> compiler]).
>
> --
> Mats
>
> > Thank you
> >
> > ~psr
> >
> >
> > --
> > ---
> > pradeep singh rautela
> >
> > "Genius is 1% inspiration, and 99% perspiration" - not me :)
>
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> http://lists.xensource.com/xen-devel



-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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