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

[Pierre Morel]

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

--
Pierre Morel
IBM Lab Boeblingen