Re: [PATCH v5 8/8] s390: guest support for diagnose 0x318

[Thomas Huth <thuth@redhat.com>]

On 11/09/2020 17.08, Thomas Huth wrote:
> On 10/09/2020 11.36, Collin Walling wrote:
>> DIAGNOSE 0x318 (diag318) is an s390 instruction that allows the storage
>> of diagnostic information that is collected by the firmware in the case
>> of hardware/firmware service events.
>>
>> QEMU handles the instruction by storing the info in the CPU state. A
>> subsequent register sync will communicate the data to the hypervisor.
>>
>> QEMU handles the migration via a VM State Description.
>>
>> This feature depends on the Extended-Length SCCB (els) feature. If
>> els is not present, then a warning will be printed and the SCLP bit
>> that allows the Linux kernel to execute the instruction will not be
>> set.
>>
>> Availability of this instruction is determined by byte 134 (aka fac134)
>> bit 0 of the SCLP Read Info block. This coincidentally expands into the
>> space used for CPU entries, which means VMs running with the diag318
>> capability may not be able to read information regarding all CPUs
>> unless the guest kernel supports an extended-length SCCB.
>>
>> This feature is not supported in protected virtualization mode.
>>
>> Signed-off-by: Collin Walling <walling@linux.ibm.com>
>> Acked-by: Janosch Frank <frankja@linux.ibm.com>
>> ---
>>  hw/s390x/sclp.c                     |  5 +++++
>>  include/hw/s390x/sclp.h             |  3 +++
>>  target/s390x/cpu.h                  |  2 ++
>>  target/s390x/cpu_features.h         |  1 +
>>  target/s390x/cpu_features_def.h.inc |  3 +++
>>  target/s390x/cpu_models.c           |  1 +
>>  target/s390x/gen-features.c         |  1 +
>>  target/s390x/kvm.c                  | 31 +++++++++++++++++++++++++++++
>>  target/s390x/machine.c              | 17 ++++++++++++++++
>>  9 files changed, 64 insertions(+)
>>
>> diff --git a/hw/s390x/sclp.c b/hw/s390x/sclp.c
>> index 87d468087b..ad5d70e14d 100644
>> --- a/hw/s390x/sclp.c
>> +++ b/hw/s390x/sclp.c
>> @@ -139,6 +139,11 @@ static void read_SCP_info(SCLPDevice *sclp, SCCB *sccb)
>>      s390_get_feat_block(S390_FEAT_TYPE_SCLP_CONF_CHAR_EXT,
>>                           read_info->conf_char_ext);
>>  
>> +    if (s390_has_feat(S390_FEAT_EXTENDED_LENGTH_SCCB)) {
>> +        s390_get_feat_block(S390_FEAT_TYPE_SCLP_FAC134,
>> +                            &read_info->fac134);
>> +    }
> 
> Wasn't this feature also possible if there are less than 240 CPUs? Or do
> I mix that up with something else? ... well, maybe it's best anyway if
> we only allow this when ELS is enabled.

Hmmm, looking at the location of fac134 (i.e. offset 134) and the
previous location of the CPU entries (i.e. offset 128), I think I just
mixed this up with something different. So please never mind this question.

 Thomas