Re: [PATCH v10 2/9] s390x/cpu topology: reporting the CPU topology to the guest

[Pierre Morel]

On 11/7/22 14:20, Janis Schoetterl-Glausch wrote:
> On Fri, 2022-10-28 at 12:00 +0200, Pierre Morel wrote:
> > On 10/27/22 22:42, Janis Schoetterl-Glausch wrote:
> > > On Wed, 2022-10-12 at 18:21 +0200, Pierre Morel wrote:
> > > > The guest can use the STSI instruction to get a buffer filled
> > > > with the CPU topology description.
> > > >
> > > > Let us implement the STSI instruction for the basis CPU topology
> > > > level, level 2.
> > > >
> > > > Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
> > > > ---
> > > >    include/hw/s390x/cpu-topology.h |   3 +
> > > >    target/s390x/cpu.h              |  48 ++++++++++++++
> > > >    hw/s390x/cpu-topology.c         |   8 ++-
> > > >    target/s390x/cpu_topology.c     | 109 ++++++++++++++++++++++++++++++++
> > > >    target/s390x/kvm/kvm.c          |   6 +-
> > > >    target/s390x/meson.build        |   1 +
> > > >    6 files changed, 172 insertions(+), 3 deletions(-)
> > > >    create mode 100644 target/s390x/cpu_topology.c
> > > >
> > > > diff --git a/include/hw/s390x/cpu-topology.h b/include/hw/s390x/cpu-topology.h
> > > > index 66c171d0bc..61c11db017 100644
> > > > --- a/include/hw/s390x/cpu-topology.h
> > > > +++ b/include/hw/s390x/cpu-topology.h
> > > > @@ -13,6 +13,8 @@
> > > >    #include "hw/qdev-core.h"
> > > >    #include "qom/object.h"
> > > >    
> > > > +#define S390_TOPOLOGY_POLARITY_H  0x00
> > > > +
> > > >    typedef struct S390TopoContainer {
> > > >        int active_count;
> > > >    } S390TopoContainer;
> > > > @@ -29,6 +31,7 @@ struct S390Topology {
> > > >        S390TopoContainer *socket;
> > > >        S390TopoTLE *tle;
> > > >        MachineState *ms;
> > > > +    QemuMutex topo_mutex;
> > > >    };
> > > >    
> > > >    #define TYPE_S390_CPU_TOPOLOGY "s390-topology"
> > > > diff --git a/target/s390x/cpu.h b/target/s390x/cpu.h
> > > > index 7d6d01325b..d604aa9c78 100644
> > > > --- a/target/s390x/cpu.h
> > > > +++ b/target/s390x/cpu.h
> > > [...]
> > > > +
> > > > +/* Maxi size of a SYSIB structure is when all CPU are alone in a container */
> > >
> > > Max or Maximum.
> > >
> > > > +#define S390_TOPOLOGY_SYSIB_SIZE (sizeof(SysIB_151x) +                         
> > > > \
> > > > +                                  S390_MAX_CPUS * (sizeof(SysIBTl_container) + 
> > > > \
> > > > +                                                   sizeof(SysIBTl_cpu)))
> > >
> > > Currently this is 16+248*3*8 == 5968 and will grow with books, drawer support to
> > > 16+248*5*8 == 9936 ...
> > >
> > > [...]
> > > > +
> > > > +void insert_stsi_15_1_x(S390CPU *cpu, int sel2, __u64 addr, uint8_t ar)
> > > > +{
> > > > +    uint64_t page[S390_TOPOLOGY_SYSIB_SIZE / sizeof(uint64_t)] = {};
> > >
> > > ... so calling this page is a bit misleading. Also why not make it a char[]?
> > > And maybe use a union for type punning.
> >
> > OK, what about:
> >
> >       union {
> >           char place_holder[S390_TOPOLOGY_SYSIB_SIZE];
> >           SysIB_151x sysib;
> >       } buffer QEMU_ALIGNED(8);
> I don't think you need the QEMU_ALIGNED since SysIB_151x already has it. Not 
> that it hurts to be
> explicit. If you declared the tle member as uint64_t[], you should get the 
> correct alignment
> automatically and can then drop the explicit one.

I find the explicit statement better. Why make it non explicit?

> Btw, [] seems to be preferred over [0], at least there is a commit doing a 
> conversion:
> f7795e4096 ("misc: Replace zero-length arrays with flexible array member 
> (automatic)")

OK

> > > > +    SysIB_151x *sysib = (SysIB_151x *) page;
> > > > +    int len;
> > > > +
> > > > +    if (s390_is_pv() || !s390_has_topology() ||
> > > > +        sel2 < 2 || sel2 > S390_TOPOLOGY_MAX_MNEST) {
> > > > +        setcc(cpu, 3);
> > > > +        return;
> > > > +    }
> > > > +
> > > > +    len = setup_stsi(sysib, sel2);
> > >
> > > This should now be memory safe, but might be larger than 4k,
> > > the maximum size of the SYSIB. I guess you want to set cc code 3
> > > in this case and return.
> >
> > I do not find why the SYSIB can not be larger than 4k.
> > Can you point me to this restriction?
>
> Says so at the top of the description of STSI:
>
> The SYSIB is 4K bytes and must begin at a 4 K-byte
> boundary; otherwise, a specification exception may
> be recognized.

Right, I guess I can not read.

So I will return CC=3 in case the length is greater than 4K


thanks,
Regards,

Pierre


--
Pierre Morel
IBM Lab Boeblingen