Re: [Qemu-devel] [qemu-s390x] [PATCH v2 2/3] s390: cpu feature for diagnose 318 andlimit max VCPUs to 247

[Collin Walling]

On 12/7/18 7:08 AM, Cornelia Huck wrote:
> On Thu,  6 Dec 2018 17:24:17 -0500
> Collin Walling <address@hidden> 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 <address@hidden>.
>> ---
>>  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?
> 

Good call. I'll run some tests on this and see what happens. I'll report
here on those results.

>>  
>>  typedef struct PSW {
>>      uint64_t mask;
> 


-- 
Respectfully,
- Collin Walling