Re: [Qemu-devel] [PATCH v3 2/2] i386/kvm: lower requirements for Hyper-V frequency MSRs exposure

[Roman Kagan]

On Fri, Mar 23, 2018 at 04:48:27PM -0300, Eduardo Habkost wrote:
> On Fri, Mar 23, 2018 at 12:45:30PM +0300, Roman Kagan wrote:
> > On Thu, Mar 22, 2018 at 03:38:13PM -0300, Eduardo Habkost wrote:
> > > It looks like the guest can still crash if we migrate
> > > "QEMU-2.12 -machine pc-2.11 -cpu ...,+hv-time" to a host running
> > > QEMU 2.11 and Linux < 4.14.
> > 
> > Indeed :(
> 
> Well, your patch fixes it by not enabling hv-frequencies by
> default on any machine-type.  Do you see any gotchas?

Only the need to add a new property in -stable.  I used to think that
was frowned upon.

> > So, taking the effort/complexity vs safety tradeoff into account, we can
> > consider an alternative approach: just add hv_frequencies (default=off)
> > cpu property to 2.12 and 2.11-stable, and ignore the cases where it's
> > run on QEMU versions without explicit control over this feature.  Would
> > it be too much against the current policy?
> > 
> 
> With this, we can just declare that QEMU v2.11.0 + Linux 4.14+
> was broken, and advise people to upgrade QEMU.
> 
> I think is the most reasonable option we have.  See my reply to
> the patch you sent.

Great, let's try going down this route.

Thanks,
Roman.