Re: [PATCH v5 6/8] s390/sclp: add extended-length sccb support for kvm guest

[Thomas Huth]

On 11/09/2020 15.41, Thomas Huth wrote:
> On 10/09/2020 11.36, Collin Walling wrote:
>> As more features and facilities are added to the Read SCP Info (RSCPI)
>> response, more space is required to store them. The space used to store
>> these new features intrudes on the space originally used to store CPU
>> entries. This means as more features and facilities are added to the
>> RSCPI response, less space can be used to store CPU entries.
>>
>> With the Extended-Length SCCB (ELS) facility, a KVM guest can execute
>> the RSCPI command and determine if the SCCB is large enough to store a
>> complete reponse. If it is not large enough, then the required length
>> will be set in the SCCB header.
>>
>> The caller of the SCLP command is responsible for creating a
>> large-enough SCCB to store a complete response. Proper checking should
>> be in place, and the caller should execute the command once-more with
>> the large-enough SCCB.
>>
>> This facility also enables an extended SCCB for the Read CPU Info
>> (RCPUI) command.
>>
>> When this facility is enabled, the boundary violation response cannot
>> be a result from the RSCPI, RSCPI Forced, or RCPUI commands.
>>
>> In order to tolerate kernels that do not yet have full support for this
>> feature, a "fixed" offset to the start of the CPU Entries within the
>> Read SCP Info struct is set to allow for the original 248 max entries
>> when this feature is disabled.
>>
>> Additionally, this is introduced as a CPU feature to protect the guest
>> from migrating to a machine that does not support storing an extended
>> SCCB. This could otherwise hinder the VM from being able to read all
>> available CPU entries after migration (such as during re-ipl).
>>
>> Signed-off-by: Collin Walling <walling@linux.ibm.com>
>> ---
> [...]
>>  /* Provide information about the configuration, CPUs and storage */
>>  static void read_SCP_info(SCLPDevice *sclp, SCCB *sccb)
>>  {
>> @@ -89,10 +112,15 @@ static void read_SCP_info(SCLPDevice *sclp, SCCB *sccb)
>>      int rnsize, rnmax;
>>      IplParameterBlock *ipib = s390_ipl_get_iplb();
>>      int required_len = SCCB_REQ_LEN(ReadInfo, machine->possible_cpus->len);
>> -    int offset_cpu = offsetof(ReadInfo, entries);
>> +    int offset_cpu = s390_has_feat(S390_FEAT_EXTENDED_LENGTH_SCCB) ?
>> +                     offsetof(ReadInfo, entries) :
>> +                     SCLP_READ_SCP_INFO_FIXED_CPU_OFFSET;
> 
> Sorry, but I'm having somewhat trouble to understand this...
> What's the difference between offsetof(ReadInfo, entries) and
> SCLP_READ_SCP_INFO_FIXED_CPU_OFFSET ? Aren't both terms resulting in the
> value 128 ?

Ah, well, the answer is clear after looking at patch 8/8 ... ReadInfo is
extended there, so offsetof(ReadInfo, entries) will result in a
different value.
Might have been better to move the above hunk into patch 8/8, but if you
want to keep it here, that's now ok for me, too.

Reviewed-by: Thomas Huth <thuth@redhat.com>