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

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Mon, Jun 11, 2012 at 3:42 PM, Mihai DonÈu <mihai.dontu@xxxxxxxxx> wrote:
> On Mon, 11 Jun 2012 16:18:04 +0200 Dario Faggioli wrote:
>> > Para-virtualization outperforms full-virtualization at the cost of
>> > compatibility.
>> >
>> So, if I can ask, do you mind explaining a bit more what you mean with
>> this sentence above? More specifically, when you say "at the cost of
>> compatibility", what are the compatibility costs you are referring to?
>>
>
> I think he's referring to the fact that paravirtualization requires
> extensive support from the guest OS. Linux and some flavors of BSD have
> such support, but it's trickier with Microsoft Windows. I think there's
> some work on that front too, but I don't know how far it got[1].

But then the argument is, "Feature X is useful, but only used by some
operating systems, so why don't we get rid of it."  That doesn't make
much sense to me, especially when "some operating systems" includes
basically every open-source OS out there. :-)

If the code to run PV guests was incredibly expensive to maintain,
that might make sense.  But it's not.  Furthermore, there are other
advantages to PV guests, including not having to have a QEMU instance
running to emulate a bunch of platform devices, and so on.

Now, it is true that a lot of things that were paravirtualized at
Xen's inception can now be virtualized with hardware.  It does make
sense to take advantage of that where we can, and there are developers
working on a "hybrid" PV mode, which does just that.

 -George

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