On 10/20/2017 10:54 AM, Christian Borntraeger 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.
...
- if (gs_allowed()) {
+ if (cpu_model_allowed()) {
if (kvm_vm_enable_cap(s, KVM_CAP_S390_GS, 0) == 0) {
cap_gs = 1;
Ok, honestly, I dislike this idea because it means we are effectively
lying now. We will happily accept a +gs cpu model with a 2.9 compat
machine when the point of compat machines is to mimick the older version
of Qemu which does not support GS. Yes, model checking will prevent the
worst side effects, namely, exploding migrations.
I'd far prefer a solution that makes host-model dependent on the machine
type. But I understand some of the backlash on that idea. Still, it
seems like the cleanest approach even if it will be more work.
With all of that said, I know we want this fixed and your patch seems to
fix the problem. So if you need an R-b...
Reviewed-by: Jason J. Herne <jjherne@xxxxxxxxxxxxxxxxxx>
Can we have a new tag? :-D
Reviewed-by-with-reservations: Jason J. Herne <jjherne@xxxxxxxxxxxxxxxxxx>
--
-- Jason J. Herne (jjherne@xxxxxxxxxxxxxxxxxx)
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