Re: [PATCH v3 2/4] s390x: kvm: topology: interception of PTF instruction

[Pierre Morel]

On 10/14/21 10:09, Pierre Morel wrote:
> On 10/13/21 11:11, Thomas Huth wrote:
> > On 13/10/2021 09.55, Pierre Morel wrote:
> > > On 10/13/21 09:25, Thomas Huth wrote:
> > > > On 16/09/2021 15.50, Pierre Morel wrote:
> > > > > When the host supports the CPU topology facility, the PTF
> > > > > instruction with function code 2 is interpreted by the SIE,
> > > > > provided that the userland hypervizor activates the interpretation
> > > > > by using the KVM_CAP_S390_CPU_TOPOLOGY KVM extension.
> > > > >
> > > > > The PTF instructions with function code 0 and 1 are intercepted
> > > > > and must be emulated by the userland hypervizor.
> > > > >
> > > > > Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
> > > > > ---
> > ...
> > > > > diff --git a/target/s390x/kvm/kvm.c b/target/s390x/kvm/kvm.c
> > > > > index 5b1fdb55c4..dd036961fe 100644
> > > > > --- a/target/s390x/kvm/kvm.c
> > > > > +++ b/target/s390x/kvm/kvm.c
> > > > > @@ -97,6 +97,7 @@
> > > > >   #define PRIV_B9_EQBS                    0x9c
> > > > >   #define PRIV_B9_CLP                     0xa0
> > > > > +#define PRIV_B9_PTF                     0xa2
> > > > >   #define PRIV_B9_PCISTG                  0xd0
> > > > >   #define PRIV_B9_PCILG                   0xd2
> > > > >   #define PRIV_B9_RPCIT                   0xd3
> > > > > @@ -362,6 +363,7 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
> > > > >       kvm_vm_enable_cap(s, KVM_CAP_S390_USER_SIGP, 0);
> > > > >       kvm_vm_enable_cap(s, KVM_CAP_S390_VECTOR_REGISTERS, 0);
> > > > >       kvm_vm_enable_cap(s, KVM_CAP_S390_USER_STSI, 0);
> > > > > +    kvm_vm_enable_cap(s, KVM_CAP_S390_CPU_TOPOLOGY, 0);
> > > >
> > > > Should this maybe rather be done in the last patch, to avoid a state 
> > > > where PTF is available, but STSI 15 is not implemented yet (when 
> > > > bisecting through these commits later)?
> > > >
> > > >   Thomas
> > >
> > > Yes you are right, thanks.
> >
> > I'm also still a little bit surprised that there is really no 
> > migration code involved here yet. What if a guest gets started on a 
> > system with KVM_CAP_S390_CPU_TOPOLOGY support and later migrated to a 
> > system without KVM_CAP_S390_CPU_TOPOLOGY support? Is there already 
> > some magic in place that rejects such a migration? If not, the guest 
> > might first learn that it could use the PTF instruction, but suddenly 
> > it is then not available anymore? Does Linux cope right with PTF 
> > becoming unavailable during runtime? But even if it does, I think it's 
> > likely not in the sense of the architecture if certain instructions 
> > might disappear during runtime? Or do I miss something?
> >
> >   Thomas
>
>
> I check on this and take the consequences.
>
> Pierre

I can use a solution using pre_save/postload migration entries to verify 
that both side of the migration use PTF and STSI_15 the same way.

Seems this direction OK ?

Regards,
Pierre

--
Pierre Morel
IBM Lab Boeblingen