Re: [PATCH v3 6/8] s390/sclp: add extended-length sccb support for kvm guest

[Thomas Huth <thuth@redhat.com>]

On 24/06/2020 14.36, Cornelia Huck wrote:
> On Thu, 18 Jun 2020 18:22:56 -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                     | 21 ++++++++++++++++++++-
>>   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, 31 insertions(+), 1 deletion(-)
>>
>> diff --git a/hw/s390x/sclp.c b/hw/s390x/sclp.c
>> index 0dfbe6e5ec..f7c49e339e 100644
>> --- a/hw/s390x/sclp.c
>> +++ b/hw/s390x/sclp.c
>> @@ -56,6 +56,18 @@ static bool sccb_has_valid_boundary(uint64_t sccb_addr, uint32_t code,
>>       uint64_t sccb_boundary = (sccb_addr & PAGE_MASK) + PAGE_SIZE;
>>   
>>       switch (code & SCLP_CMD_CODE_MASK) {
>> +    case SCLP_CMDW_READ_SCP_INFO:
>> +    case SCLP_CMDW_READ_SCP_INFO_FORCED:
>> +    case SCLP_CMDW_READ_CPU_INFO:
>> +        /*
>> +         * An extended-length SCCB is only allowed for Read SCP/CPU Info and
>> +         * is allowed to exceed the 4k boundary. The respective commands will
>> +         * set the length field to the required length if an insufficient
>> +         * SCCB length is provided.
>> +         */
>> +        if (s390_has_feat(S390_FEAT_EXTENDED_LENGTH_SCCB)) {
>> +            return true;
>> +        }
> 
> Add a fallthrough annotation?

... otherwise Coverity and friends will complain later.

>>       default:
>>           if (sccb_max_addr < sccb_boundary) {
>>               return true;
>> @@ -72,6 +84,10 @@ static bool sccb_sufficient_len(SCCB *sccb, int num_cpus, int data_len)
>>   
>>       if (be16_to_cpu(sccb->h.length) < required_len) {
>>           sccb->h.response_code = cpu_to_be16(SCLP_RC_INSUFFICIENT_SCCB_LENGTH);
>> +        if (s390_has_feat(S390_FEAT_EXTENDED_LENGTH_SCCB) &&
>> +            sccb->h.control_mask[2] & SCLP_VARIABLE_LENGTH_RESPONSE) {
>> +            sccb->h.length = required_len;
>> +        }
>>           return false;
>>       }
>>       return true;
>> @@ -101,7 +117,9 @@ static void prepare_cpu_entries(MachineState *ms, CPUEntry *entry, int *count)
>>    */
>>   static inline int get_read_scp_info_data_len(void)
>>   {
>> -    return offsetof(ReadInfo, entries);
>> +    return s390_has_feat(S390_FEAT_EXTENDED_LENGTH_SCCB) ?
>> +           offsetof(ReadInfo, entries) :
>> +           SCLP_READ_SCP_INFO_FIXED_CPU_OFFSET;
>>   }
>>   
>>   /* Provide information about the configuration, CPUs and storage */
>> @@ -116,6 +134,7 @@ static void read_SCP_info(SCLPDevice *sclp, SCCB *sccb)
>>       CPUEntry *entries_start = (void *)sccb + data_len;
>>   
>>       if (!sccb_sufficient_len(sccb, machine->possible_cpus->len, data_len)) {
>> +        warn_report("insufficient sccb size to store read scp info response");
> 
> Hm, this warning is triggered by a guest action, isn't it? Not sure how
> helpful it is.

I think this should be qemu_log_mask(LOG_GUEST_ERROR, ...) instead?

  Thomas