Re: [PATCH v4 6/8] s390/sclp: add extended-length sccb support for kvm guest

[Collin Walling]

On 6/26/20 6:01 AM, Cornelia Huck wrote:
> On Wed, 24 Jun 2020 16:23:10 -0400
> Collin Walling <walling@linux.ibm.com> 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>
>> ---
>>  hw/s390x/sclp.c                     | 24 +++++++++++++++++++++++-
>>  include/hw/s390x/sclp.h             |  1 +
>>  target/s390x/cpu_features_def.inc.h |  1 +
>>  target/s390x/gen-features.c         |  1 +
>>  target/s390x/kvm.c                  |  8 ++++++++
>>  5 files changed, 34 insertions(+), 1 deletion(-)
>>
> 
> (...)
> 
>> @@ -111,6 +131,8 @@ static void read_SCP_info(SCLPDevice *sclp, SCCB *sccb)
>>      CPUEntry *entries_start = (void *)sccb + offset_cpu;
>>  
>>      if (!sccb_verify_length(sccb, machine->possible_cpus->len, offset_cpu)) 
>> {
>> +        qemu_log_mask(LOG_GUEST_ERROR, "insufficient sccb size to store "
>> +                      "read scp info response\n");
> 
> Not sure if logging needed/provided length would be helpful here.
> 

I had the thought that it may be beneficial for kernel development -- it
gives a response if an SCCB size needs to be larger.

>>          return;
>>      }
>>  
> 
> (...)
> 
> Acked-by: Cornelia Huck <cohuck@redhat.com>
> 

Thanks.

-- 
Regards,
Collin

Stay safe and stay healthy