On 10/09/2020 11.36, 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 <wall...@linux.ibm.com>
> ---
[...]
> /* Provide information about the configuration, CPUs and storage */
> static void read_SCP_info(SCLPDevice *sclp, SCCB *sccb)
> {
> @@ -89,10 +112,15 @@ static void read_SCP_info(SCLPDevice *sclp, SCCB *sccb)
> int rnsize, rnmax;
> IplParameterBlock *ipib = s390_ipl_get_iplb();
> int required_len = SCCB_REQ_LEN(ReadInfo, machine->possible_cpus->len);
> - int offset_cpu = offsetof(ReadInfo, entries);
> + int offset_cpu = s390_has_feat(S390_FEAT_EXTENDED_LENGTH_SCCB) ?
> + offsetof(ReadInfo, entries) :
> + SCLP_READ_SCP_INFO_FIXED_CPU_OFFSET;
Sorry, but I'm having somewhat trouble to understand this...
What's the difference between offsetof(ReadInfo, entries) and
SCLP_READ_SCP_INFO_FIXED_CPU_OFFSET ? Aren't both terms resulting in the
value 128 ?
Thomas
