Re: [Qemu-devel] [PATCH 1/2] s390x/cpumodel: default enable mepoch for z14 and later

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 11 Feb 2019 09:55:25 +0100
David Hildenbrand <david@redhat.com> wrote:

> On 11.02.19 09:28, Christian Borntraeger wrote:
> > 
> > 
> > On 11.02.2019 09:23, Cornelia Huck wrote:  
> >> On Fri,  8 Feb 2019 17:11:10 -0500
> >> Collin Walling <walling@linux.ibm.com> wrote:
> >>  
> >>> Latest systems and host kernels support mepoch, which is a
> >>> feature that was meant to be supported for z14 GA1 from the
> >>> get-go. Let's copy it to the z14 GA1 default CPU model.  
> >>
> >> Do older kernels simply not support it, or was there something actively
> >> missing? IOW, is it fine to turn this on with older kernels?  
> > 
> > Older kernels will not provide the necessary implementation, so mepoch will not
> > work with such older kernel. (So worst case would be that a new qemu+old kernel
> > requires to add mepoch=off to -cpu z14)  
> 
> Or use a compat machine.
> 
> > The idea of this patch is to update QEMU in accordance with newer kernels to use
> > more features by default if people use -cpu z14.  
> 
> Agreed, this was the original purpose of the default models. Can "break"
> on old kernels, but the compat machines can help for these users, so it
> does not really "break".

That seems fine then.

> 
> > 
> > Now that I look a 2nd time: shall we also add 
> > S390_FEAT_GROUP_MULTIPLE_EPOCH_PTFF ?  
> 
> I think so.
> 
> > 
> >   
> >>
> >> [It's probably all good, although I like to wait for David's ack for
> >> any cpu model patches :)]  
> 
> I'm fine with this :)
> 
> With S390_FEAT_GROUP_MULTIPLE_EPOCH_PTFF also added
> 
> Reviewed-by: David Hildenbrand <david@redhat.com>

Ok, can I please get a v2 with that added as well?

> 
> >>  
> >>>
> >>> Machines s390-ccw-virtio-3.1 and older will retain the old CPU
> >>> models and will not provide this bit in the default model.
> >>>
> >>> Signed-off-by: Collin Walling <walling@linux.ibm.com>
> >>> ---
> >>>  hw/s390x/s390-virtio-ccw.c  | 1 +
> >>>  target/s390x/gen-features.c | 1 +
> >>>  2 files changed, 2 insertions(+)
> >>>
> >>> diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
> >>> index fd9d0b0542..32c5027345 100644
> >>> --- a/hw/s390x/s390-virtio-ccw.c
> >>> +++ b/hw/s390x/s390-virtio-ccw.c
> >>> @@ -757,6 +757,7 @@ DEFINE_CCW_MACHINE(4_0, "4.0", true);
> >>>  static void ccw_machine_3_1_instance_options(MachineState *machine)
> >>>  {
> >>>      ccw_machine_4_0_instance_options(machine);
> >>> +    s390_cpudef_featoff_greater(14, 1, S390_FEAT_MULTIPLE_EPOCH);
> >>>  }
> >>>  
> >>>  static void ccw_machine_3_1_class_options(MachineClass *mc)
> >>> diff --git a/target/s390x/gen-features.c b/target/s390x/gen-features.c
> >>> index 70015eaaf5..d258d67dca 100644
> >>> --- a/target/s390x/gen-features.c
> >>> +++ b/target/s390x/gen-features.c
> >>> @@ -550,6 +550,7 @@ static uint16_t default_GEN14_GA1[] = {
> >>>      S390_FEAT_GROUP_MSA_EXT_6,
> >>>      S390_FEAT_GROUP_MSA_EXT_7,
> >>>      S390_FEAT_GROUP_MSA_EXT_8,
> >>> +    S390_FEAT_MULTIPLE_EPOCH,
> >>>  };
> >>>  
> >>>  /* QEMU (CPU model) features */  
> >>  
> >   
> 
>