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

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

commit 015871b0edc3aa114e7bac8a9f2ee3079acb1909 upstream.

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: e1688250bd65 ("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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>