KVM: s390: introduce post handlers for STSI

The Store System Information (STSI) instruction currently collects all
information it relays to the caller in the kernel. Some information,
however, is only available in user space. An example of this is the
guest name: The kernel always sets "KVMGuest", but user space knows the
actual guest name.

This patch introduces a new exit, KVM_EXIT_S390_STSI, guarded by a
capability that can be enabled by user space if it wants to be able to
insert such data. User space will be provided with the target buffer
and the requested STSI function code.

Reviewed-by: Eric Farman <farman@linux.vnet.ibm.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Ekaterina Tumanova <tumanova@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

arch/s390/kvm/kvm-s390.c

@@ -178,6 +178,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_VM_ATTRIBUTES:
 	case KVM_CAP_MP_STATE:
 	case KVM_CAP_S390_USER_SIGP:
+	case KVM_CAP_S390_USER_STSI:
 		r = 1;
 		break;
 	case KVM_CAP_S390_MEM_OP:
@@ -280,6 +281,10 @@ static int kvm_vm_ioctl_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
 		kvm->arch.use_vectors = MACHINE_HAS_VX;
 		r = MACHINE_HAS_VX ? 0 : -EINVAL;
 		break;
+	case KVM_CAP_S390_USER_STSI:
+		kvm->arch.user_stsi = 1;
+		r = 0;
+		break;
 	default:
 		r = -EINVAL;
 		break;