Re: [Qemu-devel] [QEMU-PPC] [PATCH 4/4] target/ppc/spapr: Enable the large decrementer by default on POWER9

[Suraj Jitindar Singh <sjitindarsingh@gmail.com>]

On Tue, 2019-02-26 at 14:59 +1100, David Gibson wrote:
> On Tue, Feb 26, 2019 at 02:05:31PM +1100, Suraj Jitindar Singh wrote:
> > Enable the large decrementer by default on POWER9 cpu models. The
> > default value applied is that provided in the cpu class.
> > 
> > Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
> > ---
> >  hw/ppc/spapr_caps.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> > 
> > diff --git a/hw/ppc/spapr_caps.c b/hw/ppc/spapr_caps.c
> > index e07568fb94..f48aa367e3 100644
> > --- a/hw/ppc/spapr_caps.c
> > +++ b/hw/ppc/spapr_caps.c
> > @@ -566,11 +566,18 @@ sPAPRCapabilityInfo
> > capability_table[SPAPR_CAP_NUM] = {
> >  static sPAPRCapabilities default_caps_with_cpu(sPAPRMachineState
> > *spapr,
> >                                                 const char
> > *cputype)
> >  {
> > +    PowerPCCPUClass *pcc =
> > POWERPC_CPU_CLASS(object_class_by_name(cputype));
> >      sPAPRMachineClass *smc = SPAPR_MACHINE_GET_CLASS(spapr);
> >      sPAPRCapabilities caps;
> >  
> >      caps = smc->default_caps;
> >  
> > +    caps.caps[SPAPR_CAP_LARGE_DECREMENTER] = pcc->hdecr_bits;
> 
> So, the rule with default_caps_with_cpu() is that it can reduce the
> value from the machine-global default, but never increase it (because
> that could change guest visible behaviour for existing machine
> versions).
> 
> I think the line above will do that.

Yep

> 
> > +    if (!ppc_type_check_compat(cputype, CPU_POWERPC_LOGICAL_3_00,
> > +                               0, spapr->max_compat_pvr)) {
> > +        caps.caps[SPAPR_CAP_LARGE_DECREMENTER] = 0;
> 
> And this part I think is redundant, because hdecr_bits won't be large
> for anything pre-POWER9.
> 
> > +    }
> > +
> >      if (!ppc_type_check_compat(cputype, CPU_POWERPC_LOGICAL_2_07,
> >                                 0, spapr->max_compat_pvr)) {
> >          caps.caps[SPAPR_CAP_HTM] = SPAPR_CAP_OFF;
> 
>