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

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

On 5/25/20 6:50 AM, Janosch Frank wrote:
> 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.
> 
> 

Very cool. This is a much easier approach. Thanks for looking into it!

>>
>>> 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