Re: xennet/xbd on HVM

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Mon, Jun 14, 2010 at 10:20:01AM -0500, Dustin Marquess wrote:
> All,
> 
> Since we're probably a long way from the 'holy grail' of getting SMP
> in a NetBSD PV domU, right now we're currently using HVM and the
> performance hits that this dictates.
> 
> I'm curious on what it would take to get xennet/xbd (and I guess the
> required xenbus/hypervisor) to work in HVM mode.  This would allow the
> use of SMP, and yet bypass the hard disk/network emulation overhead.
> The FreeBSD 'XENHVM' config file does this currently.
> 
> Obviously it's not as great as true PV, but it's definitely better
> than full HVM.  Would this be somewhat simple to do, or would it be
> such a pain that we're better off just waiting for SMP in PV?

I've not looked at it in details, but I don't think it's a lot of
work. You need a driver for the pci device exposed by qemu-dm,
and attach the hypervisor to it. this probably requires a different
front-end than the one attaching to mainbus.
I would expect accesses to the xenstore to be quite similar to PV
(that is, shared pages and an interrupt).
For interrupts, these would probably be turned into soft interrupts
at some point, but I don't know how PV events show up in a HVM
guest.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--