KVM: s390: enable SIMD only when no VCPUs were created

We should never allow to enable/disable any facilities for the guest
when other VCPUs were already created.

kvm_arch_vcpu_(load|put) relies on SIMD not changing during runtime.
If somebody would create and run VCPUs and then decides to enable
SIMD, undefined behaviour could be possible (e.g. vector save area
not being set up).

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: stable@vger.kernel.org # 4.1+

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 8fe2f1c722dcabe22b3b3731bdce5e22092703db..846589281b046f414837cbd3f7ab9f4d0c675a51 100644 (file)
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -342,12 +342,16 @@ static int kvm_vm_ioctl_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
                r = 0;
                break;
        case KVM_CAP_S390_VECTOR_REGISTERS:
-               if (MACHINE_HAS_VX) {
+               mutex_lock(&kvm->lock);
+               if (atomic_read(&kvm->online_vcpus)) {
+                       r = -EBUSY;
+               } else if (MACHINE_HAS_VX) {
                        set_kvm_facility(kvm->arch.model.fac->mask, 129);
                        set_kvm_facility(kvm->arch.model.fac->list, 129);
                        r = 0;
                } else
                        r = -EINVAL;
+               mutex_unlock(&kvm->lock);
                VM_EVENT(kvm, 3, "ENABLE: CAP_S390_VECTOR_REGISTERS %s",
                         r ? "(not available)" : "(success)");
                break;