Re: [PATCH v2 6/8] s390/sclp: add extended-length sccb support for kvm guest

[Cornelia Huck <cohuck@redhat.com>]

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On Mon, 18 May 2020 10:55:24 +0200
Janosch Frank <frankja@linux.ibm.com> wrote:

> On 5/16/20 12:20 AM, 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 <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                  |  4 ++++
> >  5 files changed, 27 insertions(+), 1 deletion(-)
> > 
> > diff --git a/hw/s390x/sclp.c b/hw/s390x/sclp.c
> > index 755f5f3fab..bde4c5420e 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 allowed_len = (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 RSCPI and RSCPU and is

Nit: I had to stare at this for a bit before I figured out what the
acronyms refer to.

> > +         * 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;
> > +        }
> >      default:
> >          if (current_len <= allowed_len) {
> >              return true;

(...)

> > diff --git a/target/s390x/gen-features.c b/target/s390x/gen-features.c
> > index 8ddeebc544..6857f657fb 100644
> > --- a/target/s390x/gen-features.c
> > +++ b/target/s390x/gen-features.c
> > @@ -522,6 +522,7 @@ static uint16_t full_GEN12_GA1[] = {
> >      S390_FEAT_AP_QUEUE_INTERRUPT_CONTROL,
> >      S390_FEAT_AP_FACILITIES_TEST,
> >      S390_FEAT_AP,
> > +    S390_FEAT_EXTENDED_LENGTH_SCCB,
> >  };
> >  
> >  static uint16_t full_GEN12_GA2[] = {
> > diff --git a/target/s390x/kvm.c b/target/s390x/kvm.c
> > index 69881a0da0..380fb81822 100644
> > --- a/target/s390x/kvm.c
> > +++ b/target/s390x/kvm.c
> > @@ -2456,6 +2456,10 @@ void kvm_s390_get_host_cpu_model(S390CPUModel *model, Error **errp)
> >          KVM_S390_VM_CRYPTO_ENABLE_APIE)) {
> >          set_bit(S390_FEAT_AP, model->features);
> >      }
> > +
> > +    /* Extended-Length SCCB is handled entirely within QEMU */
> > +    set_bit(S390_FEAT_EXTENDED_LENGTH_SCCB, model->features);
> > +  
> 
> We need to fence this for secure guests as the SIDA is only 4k at the
> moment.
> 
> Do we need to take extra steps for migration safety?
> I guess this is only available with host-passthrough or -model?

What do you mean with '-model'? I think it can be added manually
everywhere? But I'm always a bit confused by cpu models.

> 
> >      /* strip of features that are not part of the maximum model */
> >      bitmap_and(model->features, model->features, model->def->full_feat,
> >                 S390_FEAT_MAX);
> >   
> 
> 


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