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



On Wed, Jul 24, 2013 at 08:30:12AM +0200, Jiri Denemark wrote:
> On Wed, Jul 24, 2013 at 13:48:24 +0800, Hu Tao wrote:
> > Added: Jiri Denemark <jdenemar 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 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.

I don't think this is a strong reason to against host-passthrough in
virt-manager. What's your opinion, Cole?


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