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

[Collin Walling <walling@linux.ibm.com>]

On 9/11/20 9:54 AM, Thomas Huth wrote:
> 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>
> 
> 

I see your point. In retrospect, it might've been better to include it
in patch 8/8 so it's more clear why these features are introduced within
the same patch set.

If there are any requests to change / fixup this patch in any other
regard, then I'll consider moving the offset_cpu calculation to 8/8.

Otherwise, I'll leave it here :)

Thanks!

-- 
Regards,
Collin

Stay safe and stay healthy