Re: [PATCH v2 3/5] s390x: topology: CPU topology objects and structures

[Pierre Morel <pmorel@linux.ibm.com>]

On 9/7/21 9:32 AM, Thomas Huth wrote:
> On 22/07/2021 19.42, Pierre Morel wrote:
>> We use new objects to have a dynamic administration of the CPU topology.
>> The highier level object is the S390 book. In a first implementation
> 

> 
> I didn't spot any migration related code in here ... is this already 
> migration-safe?
> 

Not sure at all.

The topology may change at any moment and we interpret PTF, the 
instruction which tell us if the topology changed.
Obviously the topology on the target may not be the same as on the source.

So what I propose is to disable topology change during the migration:
- on migration start, disable PTF interpretation and block the 
topology_change _report in the emulation.
- on migration end set back PTF interpretation and unblock the emulation

In the case, in discussion with David on KVM, that we do not emulate PTF 
for hosts without the stfl(11) we can even make it simpler in QEMU by 
always reporting "no change" for PTF 2 in the emulation.

Note that the Linux kernel, even if the topology can change at any 
moment use a polling every minute to check the topology changes, so I 
guess we can ignore the optimization during the migration.

What do you think?


-- 
Pierre Morel
IBM Lab Boeblingen