Re: [Xen-users] The future of para-virtualization

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Tue, Jun 12, 2012 at 5:05 PM, Niu Xinli <niuxinli1989@xxxxxxxxx> wrote:
> Thanks for all the replies. By "compatibility" I mean the guest OS must be
> modified for para-virtualization.

Don't think of paravirtualization as "on" or "off"; think about it as
a spectrum.  There are a wide range of interfaces the hypervisor
provides which can either be identical to real hardware (fully
virtualized) or designed specifically for virtualization
(paravirtualized).  Any guest OS can use as much or as little of the
PV interfaces as they want.

For instance, you can run your VM in fully virtualized mode, using
disk and network cards emulated by qemu.  Or, you can run in fully
virtualized mode, but have paravirtualized disk and network
interfaces.  You can also run in HVM mode but use paravirtualized
interrupts; there are patches in Linux to do just that, and it speeds
things up significantly.  At the moment if you choose HVM mode, you
have to do the fully virtualized boot sequence (start in 16-bit mode
with a BIOS, and boot up through 32-bit mode to 64-bit mode); but
there's no reason it has to be that way.

As long as the cost of maintaining the paravirtualized interface is
lower than the aggregate value provided to all guests that can use
that interface, I think it makes sense to keep it.  And since the cost
of maintaining the interfaces is pretty low, and the number of guests
that can use it is high, most PV interfaces will remain for some time
to come.

Does that make sense / answer your question?

 -George

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