Re: [Qemu-devel] [PATCH v2 2/3] s390: cpu feature for diagnose 318 andlimit max VCPUs to 247

[Cornelia Huck <cohuck@redhat.com>]

On Thu,  6 Dec 2018 17:24:17 -0500
Collin Walling <walling@linux.ibm.com> wrote:

> Diagnose 318 is a new z14.2 CPU feature. Since we are able to emulate
> it entirely via KVM, we can add guest support for earlier models. A
> new CPU feature for diagnose 318 (shortened to diag318) will be made
> available to guests starting with the zEC12-full CPU model.
> 
> The z14.2 adds a new read SCP info byte (let's call it byte 134) to
> detect the availability of diag318. Because of this, we have room for
> one less VCPU and thus limit the max VPUs supported in a configuration
> to 247 (down from 248).
> 
> Signed-off-by: Collin Walling <walling@linux.ibm.com>.
> ---
>  hw/s390x/sclp.c                 | 2 ++
>  include/hw/s390x/sclp.h         | 2 ++
>  target/s390x/cpu.h              | 2 +-
>  target/s390x/cpu_features.c     | 3 +++
>  target/s390x/cpu_features.h     | 1 +
>  target/s390x/cpu_features_def.h | 3 +++
>  target/s390x/gen-features.c     | 1 +
>  target/s390x/kvm.c              | 1 +
>  8 files changed, 14 insertions(+), 1 deletion(-)
> 

> diff --git a/target/s390x/cpu.h b/target/s390x/cpu.h
> index 8c2320e..594b4a4 100644
> --- a/target/s390x/cpu.h
> +++ b/target/s390x/cpu.h
> @@ -52,7 +52,7 @@
>  
>  #define MMU_USER_IDX 0
>  
> -#define S390_MAX_CPUS 248
> +#define S390_MAX_CPUS 247

Isn't that already problematic if you try to migrate from an older QEMU
with all possible vcpus defined? IOW, don't you really need a way that
older machines can still run with one more vcpu?

>  
>  typedef struct PSW {
>      uint64_t mask;