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

Eric Farman
2026-05-05T20:45:30Z

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.

Fixes: 8e8678e740ec ("KVM: s390: Add capability that forwards operation exceptions")
Signed-off-by: Eric Farman <farman@linux.ibm.com>
---
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;
}

--
2.51.0