Re: [PATCH v2 6/8] s390/sclp: add extended-length sccb support for kvm guest

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

On 5/18/20 4:55 AM, Janosch Frank wrote:
> On 5/16/20 12:20 AM, 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>
>> ---
>>  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                  |  4 ++++
>>  5 files changed, 27 insertions(+), 1 deletion(-)
>>
>> diff --git a/hw/s390x/sclp.c b/hw/s390x/sclp.c
>> index 755f5f3fab..bde4c5420e 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 allowed_len = (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 RSCPI and RSCPU 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;
>> +        }
>>      default:
>>          if (current_len <= allowed_len) {
>>              return true;
>> @@ -72,6 +84,10 @@ static bool sccb_has_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 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_has_sufficient_len(sccb, machine->possible_cpus->len, data_len)) {
>> +        warn_report("insufficient sccb size to store full read scp info response");
>>          return;
>>      }
>>  
>> diff --git a/include/hw/s390x/sclp.h b/include/hw/s390x/sclp.h
>> index 822eff4396..ef2d63eae9 100644
>> --- a/include/hw/s390x/sclp.h
>> +++ b/include/hw/s390x/sclp.h
>> @@ -110,6 +110,7 @@ typedef struct CPUEntry {
>>      uint8_t reserved1;
>>  } QEMU_PACKED CPUEntry;
>>  
>> +#define SCLP_READ_SCP_INFO_FIXED_CPU_OFFSET     128
>>  typedef struct ReadInfo {
>>      SCCBHeader h;
>>      uint16_t rnmax;
>> diff --git a/target/s390x/cpu_features_def.inc.h b/target/s390x/cpu_features_def.inc.h
>> index 60db28351d..3548d65a69 100644
>> --- a/target/s390x/cpu_features_def.inc.h
>> +++ b/target/s390x/cpu_features_def.inc.h
>> @@ -97,6 +97,7 @@ DEF_FEAT(GUARDED_STORAGE, "gs", STFL, 133, "Guarded-storage facility")
>>  DEF_FEAT(VECTOR_PACKED_DECIMAL, "vxpd", STFL, 134, "Vector packed decimal facility")
>>  DEF_FEAT(VECTOR_ENH, "vxeh", STFL, 135, "Vector enhancements facility")
>>  DEF_FEAT(MULTIPLE_EPOCH, "mepoch", STFL, 139, "Multiple-epoch facility")
>> +DEF_FEAT(EXTENDED_LENGTH_SCCB, "els", STFL, 140, "Extended-length SCCB facility")
>>  DEF_FEAT(TEST_PENDING_EXT_INTERRUPTION, "tpei", STFL, 144, "Test-pending-external-interruption facility")
>>  DEF_FEAT(INSERT_REFERENCE_BITS_MULT, "irbm", STFL, 145, "Insert-reference-bits-multiple facility")
>>  DEF_FEAT(MSA_EXT_8, "msa8-base", STFL, 146, "Message-security-assist-extension-8 facility (excluding subfunctions)")
>> diff --git a/target/s390x/gen-features.c b/target/s390x/gen-features.c
>> index 8ddeebc544..6857f657fb 100644
>> --- a/target/s390x/gen-features.c
>> +++ b/target/s390x/gen-features.c
>> @@ -522,6 +522,7 @@ static uint16_t full_GEN12_GA1[] = {
>>      S390_FEAT_AP_QUEUE_INTERRUPT_CONTROL,
>>      S390_FEAT_AP_FACILITIES_TEST,
>>      S390_FEAT_AP,
>> +    S390_FEAT_EXTENDED_LENGTH_SCCB,
>>  };
>>  
>>  static uint16_t full_GEN12_GA2[] = {
>> diff --git a/target/s390x/kvm.c b/target/s390x/kvm.c
>> index 69881a0da0..380fb81822 100644
>> --- a/target/s390x/kvm.c
>> +++ b/target/s390x/kvm.c
>> @@ -2456,6 +2456,10 @@ void kvm_s390_get_host_cpu_model(S390CPUModel *model, Error **errp)
>>          KVM_S390_VM_CRYPTO_ENABLE_APIE)) {
>>          set_bit(S390_FEAT_AP, model->features);
>>      }
>> +
>> +    /* Extended-Length SCCB is handled entirely within QEMU */
>> +    set_bit(S390_FEAT_EXTENDED_LENGTH_SCCB, model->features);
>> +
> 
> We need to fence this for secure guests as the SIDA is only 4k at the
> moment.
> 

I don't know much about the SE stuff, so I'll take your word for it.
Should this follow the same fencing as diag 318 and simply check for pv
mode?

> Do we need to take extra steps for migration safety?
> I guess this is only available with host-passthrough or -model?
> 
>>      /* strip of features that are not part of the maximum model */
>>      bitmap_and(model->features, model->features, model->def->full_feat,
>>                 S390_FEAT_MAX);
>>
> 
> 


-- 
--
Regards,
Collin

Stay safe and stay healthy