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Re: [Xen-users] OS kernels ports, VT, Pacifica & performance

Sylvain Coutant wrote:
Hi all,

First, I'm not sure this post should have been sent to xen-devel or here. 
Please, any list owner : forward to xen-devel if you read this and feel it 
should have gone there.

I wonder what will be the advantage, in terms of performance, of having 
optimized kernels for XenU when VT/Pacifica will be there.

AFAIK, using "standard" kernels means emulating peripherals (network card and so on) on dom0. Xen 
"optimized" or "ported" kernels should have a performance advantage. But has this perf 
increase already been evaluated ?

Question behind this : does it worth the work to port some other OSes to Xen 
architecture ?

Not having access to Pacifica/VT hardware (I'd almost kill for it), there are limits to what can be commented.

Think of normal multitasking. Handing over the cpu timeslice from one running process to another means saving the running process' state and loading another sleeping process' state.

With virtualization, the whole OS state and pagetable structure needs to be saved and revoked. A bit more timeconsuming..
Having virtualisation hardware support for this will be a real speed-up.
Having hardware support in a cpu with onchip memorycontroller will mean even more speed-up.

Without that onchip memorycontroller, quite a lot still needs be done in software. Yes, the cpu 'hardware' virtualization is not just a mix of registers and logic, but also firmware; however, still rather faster than what the target systems OS + virtualization mechanism can achieve.

With current virtualization techniques, guest/domU systems are always emulated, so having cpu hardware virtualization doesn't really change that, AFAICS. It'll mean the abilility to run unpatched OS's, though.

It's another ballgame further into the future, when the whole platform and PCIe gets increasingly virtualized. Maybe sometime around 2008..

Kind regards,
Mogens Valentin

PCIe virtualisation: Imagine cat herding with a firehose
and firecrackers.  That is notably easier than getting all
the peripheral makers to play along.
  -- fun on theinquirer.net

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