Re: [Qemu-devel] [PATCH v2 13/19] target/s390x: use "core-id" for cpu number/address/id handling

[Matthew Rosato <mjrosato@linux.vnet.ibm.com>]

On 09/04/2017 11:43 AM, David Hildenbrand wrote:
> Some time ago we discussed that using "id" as property name is not the
> right thing to do, as it is a reserved property for other devices and
> will not work with device_add.
> 
> Switch to the term "core-id" instead, and use it as an equivalent to
> "CPU address" mentioned in the PoP. There is no such thing as cpu number,
> so rename env.cpu_num to env.core_id. We use "core-id" as this is the
> common term to use for device_add later on (x86 and ppc).
> 
> We can get rid of cpu->id now. Keep cpu_index and env->core_id in sync.
> cpu_index was already implicitly used by e.g. cpu_exists(), so keeping
> both in sync seems to be the right thing to do.
> 

Rename & removal of 'id' is fine w/ me - in retrospect, it's a shame we
used 'id' in the first place.

> cpu_index will now no longer automatically get set via
> cpu_exec_realizefn(). For now, we were lucky that both implicitly stayed
> in sync.>

Good catch.

> Our new cpu property "core-id" can be a static property. Range checks can
> be avoided by using the correct type and the "setting after realized"
> check is done implicitly.
> 
> device_add will later need the reserved "id" property. Hotplugging a CPU
> on s390x will then be: "device_add host-s390-cpu,id=cpu2,core-id=2".
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---

[...]

> diff --git a/target/s390x/translate.c b/target/s390x/translate.c
> index 4b0db7b7bd..b8963f2fe2 100644
> --- a/target/s390x/translate.c
> +++ b/target/s390x/translate.c
> @@ -3822,10 +3822,7 @@ static ExitStatus op_ssm(DisasContext *s, DisasOps *o)
>  static ExitStatus op_stap(DisasContext *s, DisasOps *o)
>  {
>      check_privileged(s);
> -    /* ??? Surely cpu address != cpu number.  In any case the previous
> -       version of this stored more than the required half-word, so it
> -       is unlikely this has ever been tested.  */
> -    tcg_gen_ld32u_i64(o->out, cpu_env, offsetof(CPUS390XState, cpu_num));
> +    tcg_gen_ld32u_i64(o->out, cpu_env, offsetof(CPUS390XState, core_id));
>      return NO_EXIT;
>  }
> 

Are you sure it's OK to remove this blurb in its entirety?  You are
certainly collapsing the various CPU identifiers, but you aren't
changing the size of the store from when this blurb was put in
(411fea3d) So, "the previous version of this stored more than the
required half-word" seems to be still relevant -- Unless you've gone
ahead and tested it out?

Outside of that nit, I like the changes.

Reviewed-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>