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

[Pierre Morel]

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