Re: [PATCH] KVM: s390: Toggle operation exception for userspace

[Eric Farman <farman@linux.ibm.com>]

On Wed, 2026-05-06 at 15:19 +0200, Claudio Imbrenda wrote:
> On Tue,  5 May 2026 22:45:21 +0200
> Eric Farman <farman@linux.ibm.com> wrote:
> 
> > The KVM_CAP_S390_USER_OPEREXEC capability allows operation exceptions
> > to be forwarded to userspace. But the actual enablement at the hardware
> > level occurs in kvm_arch_vcpu_postcreate(), and only if STFLE.74 or
> > user_instr0 are enabled. The latter is associated with a separate
> > capability (KVM_CAP_S390_USER_INSTR0), so the only way this happens
> > for the USER_OPEREXEC capability is if STFLE.74 is enabled. KVM
> > unconditionally enables this bit in kvm_arch_init_vm(), but the guest
> > could disable it from the CPU model and thus ignore this capability.
> > 
> > Add USER_OPEREXEC to the check in kvm_arch_vcpu_postcreate(), such
> > that either capability would enable this type of exception.
> > KVM_CAP_S390_USER_INSTR0
> > Fixes: 8e8678e740ec ("KVM: s390: Add capability that forwards
> > operation exceptions") Signed-off-by: Eric Farman
> > <farman@linux.ibm.com>
> 
> Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> 
> can you also extend the existing selftest for KVM_CAP_S390_USER_OPEREXEC
> to also properly test with all combinations of
> STFLE.74 / KVM_CAP_S390_USER_OPEREXEC / KVM_CAP_S390_USER_INSTR0

Ah, of course. Will do.

> 
> > ---
> >  arch/s390/kvm/kvm-s390.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> > index e09960c2e6ed..a91a42174302 100644
> > --- a/arch/s390/kvm/kvm-s390.c
> > +++ b/arch/s390/kvm/kvm-s390.c
> > @@ -3521,7 +3521,8 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
> >  		vcpu->arch.gmap = vcpu->kvm->arch.gmap;
> >  		sca_add_vcpu(vcpu);
> >  	}
> > -	if (test_kvm_facility(vcpu->kvm, 74) || vcpu->kvm->arch.user_instr0)
> > +	if (test_kvm_facility(vcpu->kvm, 74) || vcpu->kvm->arch.user_instr0 ||
> > +	    vcpu->kvm->arch.user_operexec)
> >  		vcpu->arch.sie_block->ictl |= ICTL_OPEREXC;
> >  }
> >