Re: [PATCH v8 08/12] s390x/cpu_topology: implementing numa for the s390x topology

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

On 7/15/22 11:11, Janis Schoetterl-Glausch wrote:
> On 7/14/22 22:17, Pierre Morel wrote:
>>
>>
>> On 7/14/22 16:57, Janis Schoetterl-Glausch wrote:
>>> On 6/20/22 16:03, Pierre Morel wrote:
>>>> S390x CPU Topology allows a non uniform repartition of the CPU
>>>> inside the topology containers, sockets, books and drawers.
>>>>
>>>> We use numa to place the CPU inside the right topology container
>>>> and report the non uniform topology to the guest.
>>>>
>>>> Note that s390x needs CPU0 to belong to the topology and consequently
>>>> all topology must include CPU0.
>>>>
>>>> We accept a partial QEMU numa definition, in that case undefined CPUs
>>>> are added to free slots in the topology starting with slot 0 and going
>>>> up.
>>>
>>> I don't understand why doing it this way, via numa, makes sense for us.
>>> We report the topology to the guest via STSI, which tells the guest
>>> what the topology "tree" looks like. We don't report any numa distances to the guest.
>>> The natural way to specify where a cpu is added to the vm, seems to me to be
>>> by specify the socket, book, ... IDs when doing a device_add or via -device on
>>> the command line.
>>>
>>> [...]
>>>
>>
>> It is a choice to have the core-id to determine were the CPU is situated in the topology.
>>
>> But yes we can chose the use drawer-id,book-id,socket-id and use a core-id starting on 0 on each socket.
>>
>> It is not done in the current implementation because the core-id implies the socket-id, book-id and drawer-id together with the smp parameters.
>>
>>
> Regardless of whether the core-id or the combination of socket-id, book-id .. is used to specify where a CPU is
> located, why use the numa framework and not just device_add or -device ?

You are right, at least we should be able to use both.
I will work on this.

> 
> That feels way more natural since it should already just work if you can do hotplug.
> At least with core-id and I suspect with a subset of your changes also with socket-id, etc.

yes, it already works with core-id

> 
> Whereas numa is an awkward fit since it's for specifying distances between nodes, which we don't do,
> and you have to use a hack to get it to specify which CPUs to plug (via setting arch_id to -1).
> 

Is it only for this?

-- 
Pierre Morel
IBM Lab Boeblingen