Re: [libvirt] [PATCH v4 3/8] libxl: error out on not supported CPU mode, instead of silently ignoring
On Mon, Feb 26, 2018 at 09:48:14PM +0100, Marek Marczykowski-Górecki wrote:
On Mon, Feb 26, 2018 at 01:20:49PM -0700, Jim Fehlig wrote:
> On 02/15/2018 02:47 PM, Marek Marczykowski-Górecki wrote:
> > On Tue, Feb 13, 2018 at 09:02:35AM -0700, Jim Fehlig wrote:
> > > It looks like we never answered my question from V3, i.e. should we
change
> > > the default mode in the post-parse callback if NUMA or CPU features are
> > > defined within <cpu>?
> >
> > Hmm, but this means changing the config behind user's back, no?
>
> Well, sort of. But it is really just making the implicit explicit.
>
> > You have disabled nested HVM, upgrade, now you have enabled.
> > Global switch is some consolation here, but is it enough?
>
> I _think_ so. With a global default that disables nesting, are there any
> scenarios you envision where a previously disabled nesting becomes enabled
> after upgrade?
Probably also importing VM from older libvirt. Not sure about migration?
Any other opinion about this? Daniel? Since introduction of global
switch I'm fine with either.
As a general rule in libvirt we aim to preserve compatibility such that
upgrading either libvirt or the hypervisor stack will not change guest
visible ABI. This ensures that migration from old to new libvirt and/or
old to new hypervisor will succeed without breaking guests.
The caveat is that for QEMU driver this is only guaranteed if QEMU is
using a versioned machine type - which means x86, aarch64 and ppc only
at this time.
If previous versions of the libxl driver defaulted to having svm/vmx
enabled by default, and enw versions of libvirt libxl defualt to
disabled, that would be a potentially guest visible change. I think
you can probably argue that this is none the less OK, because there
is a global config parameter to change the default behaviour to regain
strict compatibilty
Regards,
Daniel
--
|: https://berrange.com -o- https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org -o- https://fstop138.berrange.com :|
|: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|