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

[Collin Walling]

On 6/24/20 8:55 AM, Cornelia Huck wrote:
> On Wed, 24 Jun 2020 14:40:58 +0200
> Thomas Huth <thuth@redhat.com> wrote:
> 
>> 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.
> 
> Nod.
> 

Something simple like...

/* without this feature, these commands must respect the 4k boundary */

?

>>
>>>>       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?
> 
> Yes, that sounds better.
> 
> 

Sure, sounds good.

-- 
Regards,
Collin

Stay safe and stay healthy