Re: [PATCH v4 6/8] s390/sclp: add extended-length sccb support for kvm guest

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 24 Jun 2020 16:23:10 -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                     | 24 +++++++++++++++++++++++-
>  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, 34 insertions(+), 1 deletion(-)
> 

(...)

> @@ -111,6 +131,8 @@ static void read_SCP_info(SCLPDevice *sclp, SCCB *sccb)
>      CPUEntry *entries_start = (void *)sccb + offset_cpu;
>  
>      if (!sccb_verify_length(sccb, machine->possible_cpus->len, offset_cpu)) {
> +        qemu_log_mask(LOG_GUEST_ERROR, "insufficient sccb size to store "
> +                      "read scp info response\n");

Not sure if logging needed/provided length would be helpful here.

>          return;
>      }
>  

(...)

Acked-by: Cornelia Huck <cohuck@redhat.com>