Re: [Xen-users] para- and full-virtualization on same system?

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> Mark wrote:
> > These would need to run in an HVM (full virt) domain, since OpenBSD
> > doesn't have support for Xen. I'm not sure it's as well tested as
> > Linux / Windows under HVM, though,
>
> Yikes! Do the fully-virtualized OSs need to be "well tested" - I thought
> the idea was that the OS would just work...or do you mean that the OS
> may not recognize some of the devices being provided by the virtual
> machine?

Well... good question.  The idea is indeed that the OS will Just Work.  But 
there's always the possibility that a new OS will exercise some unusual / 
undocumented feature of the hardware (or just something that hasn't been 
implemented because nobody else uses it) in which case it might break the 
emulation.  I'd say OpenBSD has a reasonably good chance of working, and it's 
quite possible somebody out there tests it - but the main regression tests 
that are reported on the xen-devel list are for Windows XP and Linux 2.6.

Also, bear in mind that the HVM feature is much newer than other parts of Xen 
and has seen less real-world use due to the requirement for special hardware.  
It's being continually regression tested and developed - you might like to 
check out the xen-devel archives for the regular test results, etc.

> > Just 64-on-64 for now - for paravirt domains, the guest interface must
> > match
> > the hypervisor. A compatability layer *may* appear at some point, but
> > it's
> > not there yet.
>
> I know that there are many optimizations yet to be made, but given
> current implementations, is there any performance comparisons between
> running a 32-bit xen-aware OS para-virtualized versus full-virtualized?
> For instance, its been said that a para-virtualized guest OS may be 3%
> slower than normal - how much slower would the same guest be if running
> fully-virtualized?

I'm not sure there are numbers for full virtualisation yet.

Performance for CPU/memory-intensive stuff should at least be fair-to-good 
(but I don't know exactly how good).  Performance for IO is likely to be 
somewhat lower due to the limitations of the current device model - there are 
plans to fix this with a new device emulation model and (at some point) with 
Xen-aware drivers.

Nb. full virt domains can't be live migrated or suspended just yet.  This will 
also come with the new device model.

Cheers,
Mark

-- 
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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