Re: [Qemu-devel] [PATCH RFC 2/4] target-ppc: with MTTCG report more threads

[Greg Kurz <groug@kaod.org>]

On Fri, 02 Sep 2016 15:04:47 +0530
Nikunj A Dadhania <nikunj@linux.vnet.ibm.com> wrote:

> Greg Kurz <groug@kaod.org> writes:
> 
> > On Fri,  2 Sep 2016 12:02:54 +0530
> > Nikunj A Dadhania <nikunj@linux.vnet.ibm.com> wrote:
> >  
> >> Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
> >> ---  
> >
> > Shouldn't this patch be the last one, when all other issues have been addressed ?
> >  
> >>  target-ppc/kvm.c     | 2 +-
> >>  target-ppc/kvm_ppc.h | 2 +-
> >>  2 files changed, 2 insertions(+), 2 deletions(-)
> >> 
> >> diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
> >> index dcb68b9..20eb450 100644
> >> --- a/target-ppc/kvm.c
> >> +++ b/target-ppc/kvm.c
> >> @@ -2090,7 +2090,7 @@ void kvmppc_set_mpic_proxy(PowerPCCPU *cpu, int mpic_proxy)
> >>  
> >>  int kvmppc_smt_threads(void)
> >>  {
> >> -    return cap_ppc_smt ? cap_ppc_smt : 1;
> >> +    return cap_ppc_smt ? cap_ppc_smt : 8;  
> >
> > If KVM is there but does not support SMT processor modes, it looks
> > wrong to return anything but 1. This check needs kvm_enabled().  
> 
> This also gets called when emulating PPC on PPC. 
> 

Yes and the current value of 1 is the default for non HV KVM and TCG. If
you want to change the default for MTTCG, then you need separate paths.

> > Also, why 8 ? This depends on the CPU model.  
> 
> Not sure if I need to emulate according to the host cpu model. I had
> selected 8, as that was the highest number of threads possible for POWER.
> 

If running in full emulation and cpu type is POWER7, this shouldn't be
higher than 4.

In the end, something like:

int kvmppc_smt_threads(void)
{
    if (kvm_enabled()) {
        return cap_ppc_smt ? cap_ppc_smt : 1;
    } else {
        return 8_or_4_or_2_or_1_depending_on_the_cpu_model;
    }
}

Cheers.

--
Greg

> > Also, since real HW allows to choose the SMT mode, maybe this should
> > be configurable from the command line as well.
> >  
> >>  }
> >>  
> >>  #ifdef TARGET_PPC64
> >> diff --git a/target-ppc/kvm_ppc.h b/target-ppc/kvm_ppc.h
> >> index 5461d10..053db0a 100644
> >> --- a/target-ppc/kvm_ppc.h
> >> +++ b/target-ppc/kvm_ppc.h
> >> @@ -128,7 +128,7 @@ static inline void kvmppc_set_mpic_proxy(PowerPCCPU *cpu, int mpic_proxy)
> >>  
> >>  static inline int kvmppc_smt_threads(void)
> >>  {
> >> -    return 1;
> >> +    return 8;  
> >
> > Same remark.
> >  
> >>  }
> >>  
> >>  static inline int kvmppc_or_tsr_bits(PowerPCCPU *cpu, uint32_t tsr_bits)  
> 
> Regards
> Nikunj
>