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

[Cornelia Huck <cohuck@redhat.com>]

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?

>      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.

>          return;
>      }
>  

(...)

Otherwise, looks good to me.