KVM: s390: vsie: Fix the initialization of the epoch extension (epdx) field

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2140899
Upstream Status: https://git.kernel.org/pub/scm/virt/kvm/kvm.git

commit 0dd4cdc
Author: Thomas Huth <thuth@redhat.com>
Date:   Wed Nov 23 10:08:33 2022 +0100

    KVM: s390: vsie: Fix the initialization of the epoch extension (epdx) field

    We recently experienced some weird huge time jumps in nested guests when
    rebooting them in certain cases. After adding some debug code to the epoch
    handling in vsie.c (thanks to David Hildenbrand for the idea!), it was
    obvious that the "epdx" field (the multi-epoch extension) did not get set
    to 0xff in case the "epoch" field was negative.
    Seems like the code misses to copy the value from the epdx field from
    the guest to the shadow control block. By doing so, the weird time
    jumps are gone in our scenarios.

    Link: https://bugzilla.redhat.com/show_bug.cgi?id=2140899
    Fixes: 8fa1696 ("KVM: s390: Multiple Epoch Facility support")
    Signed-off-by: Thomas Huth <thuth@redhat.com>
    Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
    Acked-by: David Hildenbrand <david@redhat.com>
    Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
    Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
    Cc: stable@vger.kernel.org # 4.19+
    Link: https://lore.kernel.org/r/20221123090833.292938-1-thuth@redhat.com
    Message-Id: <20221123090833.292938-1-thuth@redhat.com>
    Signed-off-by: Janosch Frank <frankja@linux.ibm.com>

Signed-off-by: Thomas Huth <thuth@redhat.com>

Author: huth

arch/s390/kvm/vsie.c

@@ -546,8 +546,10 @@ static int shadow_scb(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
 	if (test_kvm_cpu_feat(vcpu->kvm, KVM_S390_VM_CPU_FEAT_CEI))
 		scb_s->eca |= scb_o->eca & ECA_CEI;
 	/* Epoch Extension */
-	if (test_kvm_facility(vcpu->kvm, 139))
+	if (test_kvm_facility(vcpu->kvm, 139)) {
 		scb_s->ecd |= scb_o->ecd & ECD_MEF;
+		scb_s->epdx = scb_o->epdx;
+	}
 
 	/* etoken */
 	if (test_kvm_facility(vcpu->kvm, 156))