[virt-tools-list] [PATCH v2 3/3] Add support for cpu host-passthrough mode

[Jiri Denemark]

On Wed, Jul 24, 2013 at 13:48:24 +0800, Hu Tao wrote:
> Added: Jiri Denemark <jdenemar at redhat.com>
> 
> On Sun, Apr 14, 2013 at 01:45:35PM -0400, Cole Robinson wrote:
> > On 04/12/2013 05:50 AM, Guannan Ren wrote:
> > > On 04/11/2013 01:45 PM, Hu Tao wrote:
> > >> From: Ken ICHIKAWA <ichikawa.ken at jp.fujitsu.com>
> > >>
> > >> We couldn't use host-passthrough mode for virtual cpu from
> > >> virt-manager so far. This patch enables virt-manager to
> > >> configure host-passthrough mode.
...
> > So here I'm not sure we even want host-passthrough in the virt-manager UI. I
> > know it's valuable and useful, but there's the problem that libvirt 'taints'
> > the VM when you set this flag. This doesn't really have any functional effect,
> > but it basically means that libvirt devs consider this option to be not all
> > that supportable.
> 
> Yes. But what if user knows the risk and just want the option? We can
> explicitly state that this option may not be well supported.

Yeah, the reason for tainting the domain is that the environment is
generally unreproducible on another host and thus it complicates
investigation if something breaks. By tainting such domain, libvirt
basically says the user is on their own and if it doesn't work and the
failure cannot be reproduced without -cpu host, we may refuse to deal
with it (depending on the nature of that failure of course).

> >                   Exposing it in the UI seems like going against libvirt's wishes.
> 
> libvirt supports it in xml, so I don't think this is a problem.

>From libvirt's POV I don't see a reason for virt-manager not to allow
this configuration if it explicitly states that doing so is fragile, the
domain may crash and burn after migration, etc. So it really depends if
that is something which fits into virt-manager's goals or not.

> > It's also quite hard to explain host-model vs host-passthrough to an end user.
> > We could just throw them in a combo box, but I guarantee it will have people
> > asking over and over what the difference between host-model and
> > host-passthrough is.

Which might actually be the real reason for not allowing
host-passthrough to be set using virt-manager.

Jirka