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

[Thomas Huth]

On 07/09/2021 14.45, Pierre Morel wrote:
> 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?

I don't have much clue, this topology stuff is still mostly a black box to 
me - so there is no interrupt or something similar presented to the guest 
when the topology changes? The guest really has to poll for changes? ... 
that sounds like a weird design to me... if the guest polls too frequently, 
it wastes cycles due to the polling - but if it polls not often enough, it 
could run for a while with wrong topology information?

 Thomas