Re: [Qemu-stable] [PATCH for-2.12 2/2] i386/hyperv: error out if features requested but unsupported

[Roman Kagan]

On Fri, Mar 23, 2018 at 04:56:21PM -0300, Eduardo Habkost wrote:
> On Fri, Mar 23, 2018 at 03:58:08PM +0300, Roman Kagan wrote:
> > In order to guarantee compatibility on migration, QEMU should have
> > complete control over the features it announces to the guest via CPUID.
> > 
> > However, for a number of Hyper-V-related cpu properties, if the
> > corresponding feature is not supported by the underlying KVM, the
> > propery is silently ignored and the feature is not announced to the
> > guest.
> > 
> > Refuse to start with an error instead.
> > 
> > Cc: address@hidden
> > Signed-off-by: Roman Kagan <address@hidden>
> 
> I wonder if we should make these just warnings on -stable, and
> make them fatal errors only on 2.12.  I wouldn't want to make
> existing running VMs not runnable on a stable update.

OK let's follow your approach and consider who would suffer more:

a) users who started a VM on a newer kernel and then migrated to an
   older one, and got a guest crash on an unsupported MSR (I'm not even
   sure they'd be able to see the warnings)

b) users who had a VM configuration with features which didn't actually
   work (but that wasn't apparently a problem for them), and suddenly
   couldn't start their VM after QEMU upgrade (the problem is not only
   cold-restarting per se, but also live migration to the upgraded
   version: dunno if the management layer would allow to adjust the VM
   config to migrate successfully).

I don't have a strong opinion on this, will follow whatever direction
you advise.


> >  target/i386/kvm.c | 25 +++++++++++++++++++++----
> >  1 file changed, 21 insertions(+), 4 deletions(-)
> > 
> > diff --git a/target/i386/kvm.c b/target/i386/kvm.c
> > index fb20ff18c2..c9c359241c 100644
> > --- a/target/i386/kvm.c
> > +++ b/target/i386/kvm.c
> > @@ -658,17 +658,34 @@ static int hyperv_handle_properties(CPUState *cs)
> >          env->features[FEAT_HYPERV_EAX] |= HV_ACCESS_FREQUENCY_MSRS;
> >          env->features[FEAT_HYPERV_EDX] |= HV_FREQUENCY_MSRS_AVAILABLE;
> >      }
> > -    if (cpu->hyperv_crash && has_msr_hv_crash) {
> > +    if (cpu->hyperv_crash) {
> > +        if (!has_msr_hv_crash) {
> > +            fprintf(stderr,
> > +                    "Hyper-V crash MSRs are not supported by kernel\n");
> 
> I would mention the corresponding "hv-..." -cpu option
> explicitly, for clarity.

This sounds like a good idea, but I wonder if it should better be done
uniformly for all hv_* options, together with transitioning to
error_report(), and whether we want to do this at this point in the
release cycle.

Thanks,
Roman.