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

[Janosch Frank <frankja@linux.ibm.com>]

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On 5/18/20 4:31 PM, Collin Walling wrote:
> 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?

So I had another look into it and I take everything back I said:
* The stfle bit is controlled by the UV, so it's not indicated to the
guest in PV mode
* If the guest tries to execute a long SCCB the UV will return a
boundary violation error on its own (well after a notification exit anyway).

Let's therefore throw in a comment:
For PV guests this is completely fenced by the Ultravisor as Service
Call error checking and STFLE interpretation are handled by SIE.


> 
>> 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);
>>>
>>
>>
> 
> 



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