Re: [PATCH v3 4/4] spapr: Forbid nested KVM-HV in pre-power9 compat mode

[Greg Kurz <groug@kaod.org>]

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On Sat, 13 Jun 2020 17:18:04 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Thu, Jun 11, 2020 at 03:40:33PM +0200, Greg Kurz wrote:
> > Nested KVM-HV only works on POWER9.
> > 
> > Signed-off-by: Greg Kurz <groug@kaod.org>
> > Reviewed-by: Laurent Vivier <lvivier@redhat.com>
> 
> Hrm.  I have mixed feelings about this.  It does bring forward an
> error that we'd otherwise only discover when we try to load the kvm
> module in the guest.
> 
> On the other hand, it's kind of a layering violation - really it's
> KVM's business to report what it can and can't do, rather than having
> qemu anticipate it.
> 

Agreed and it seems that we can probably get KVM to report that
already. I'll have closer look.

> Allowing POWER8 compat for an L2 is something we hope to have in the
> fairly near future.

Ok but I guess we don't want to start an L2 in compat POWER8 mode
with cap-nested-hv=on, do we ?

>  Allowing POWER8 compat for L1, which is what this
> covers, is, I'll admit, likely to never happen.
> 
> 
> > ---
> >  HW/ppc/spapr_caps.c |   10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git a/hw/ppc/spapr_caps.c b/hw/ppc/spapr_caps.c
> > index 27cf2b38af27..dfe3b419daaa 100644
> > --- a/hw/ppc/spapr_caps.c
> > +++ b/hw/ppc/spapr_caps.c
> > @@ -391,6 +391,8 @@ static void cap_nested_kvm_hv_apply(SpaprMachineState *spapr,
> >                                      uint8_t val, Error **errp)
> >  {
> >      ERRP_AUTO_PROPAGATE();
> > +    PowerPCCPU *cpu = POWERPC_CPU(first_cpu);
> > +
> >      if (!val) {
> >          /* capability disabled by default */
> >          return;
> > @@ -400,6 +402,14 @@ static void cap_nested_kvm_hv_apply(SpaprMachineState *spapr,
> >          error_setg(errp, "No Nested KVM-HV support in TCG");
> >          error_append_hint(errp, "Try appending -machine cap-nested-hv=off\n");
> >      } else if (kvm_enabled()) {
> > +        if (!ppc_check_compat(cpu, CPU_POWERPC_LOGICAL_3_00, 0,
> > +                              spapr->max_compat_pvr)) {
> > +            error_setg(errp, "Nested KVM-HV only supported on POWER9");
> > +            error_append_hint(errp,
> > +                              "Try appending -machine max-cpu-compat=power9\n");
> > +            return;
> > +        }
> > +
> >          if (!kvmppc_has_cap_nested_kvm_hv()) {
> >              error_setg(errp,
> >                         "KVM implementation does not support Nested KVM-HV");
> > 
> > 
> 


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