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

[Cornelia Huck]

On Wed, 15 Jul 2020 11:35:06 -0400
Collin Walling <walling@linux.ibm.com> wrote:

> On 6/26/20 6:01 AM, Cornelia Huck wrote:
> > 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.
> >   
> 
> I had the thought that it may be beneficial for kernel development -- it
> gives a response if an SCCB size needs to be larger.

If we have the info, we might as well log it, I guess.