Re: [Qemu-devel] [PATCH/QEMU] s390x/kvm: use cpu_model_available for guarded storage on compat machines

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 20 Oct 2017 16:54:37 +0200
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> Starting a guest with
>    <os>
>     <type arch='s390x' machine='s390-ccw-virtio-2.9'>hvm</type>
>   </os>
>   <cpu mode='host-model'/>
> 
> on an IBM z14 results in
> 
> "qemu-system-s390x: Some features requested in the CPU model are not
> available in the configuration: gs"
> 
> This is because guarded storage is fenced for compat machines that did not have
> guarded storage support, but libvirt expands the cpu model according to the
> latest available machine.
> 
> While this prevents future migration abort (by not starting the guest at all),
> not being able to start a "host-model" guest is very much unexpected.  As it
> turns out, even if we would modify libvirt to not expand the cpu model to
> contain "gs" for compat machines, it cannot guarantee that a migration will
> succeed. For example if the kernel changes its features (or the user has
> nested=1 on one host but not on the other) the migration will fail
> nevertheless.  So instead of fencing "gs" for machines <= 2.9 lets allow it for
> all machine types that support the CPU model. This will make "host-model"
> runnable all the time, while relying on the CPU model to reject invalid
> migration attempts.
> 
> Suggested-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> ---
>  hw/s390x/s390-virtio-ccw.c         | 8 --------
>  include/hw/s390x/s390-virtio-ccw.h | 3 ---
>  target/s390x/kvm.c                 | 2 +-
>  3 files changed, 1 insertion(+), 12 deletions(-)

Acked-by: Cornelia Huck <cohuck@redhat.com>