Re: [PATCH v4 0/1] numa: add 'spm' option for Specific Purpose Memory

[Gregory Price <gourry@gourry.net>]

On Tue, Dec 30, 2025 at 10:55:02AM +0800, Huang, FangSheng (Jerry) wrote:
> Hi Gregory,
> 
> > Sorry i've missed prior versions, is numa the right place to put this,
> > considering that the node is not necessarily 100% SPM on a real system?
> > 
> 
> The decision to add `spm=` to NUMA rather than the memory backend was based
> on
> earlier feedback from David during our initial RFC discussions.
> 
> David raised a concern that if we put the spm flag on the memory backend, a
> user
> could accidentally pass such a memory backend to DIMM/virtio-mem/boot
> memory,
> which would have very undesired side effects.
> 

This makes sense, and in fact I almost wonder if we should actually
encode a warning in linux in general if a signal NUMA node contains
both normal and SPM.  That would help drive consistency between QEMU/KVM
and real platforms from the direction of linux.

> > (in practice it should be, but not technically required to be)
> 
> You're right that on a real system, a NUMA node is not technically required
> to
> be 100% SPM. However, in AMD's use case, the entire NUMA node memory (backed
> by
> memdev) is intended to be SPM, and this approach provides a cleaner and
> safer
> configuration interface.
> 

I figured this was the case, and honestly this just provides more
evidence that any given NUMA node probably should only have 1 "type" of
memory (or otherwise stated: uniform access within a node, non-uniform
across nodes).

---

bit of an aside - but at LPC we also talked about SPM NUMA nodes:
https://lore.kernel.org/linux-mm/20251112192936.2574429-1-gourry@gourry.net/

Would be cool to be able to detect this in the drivers and have hotplug
automatically mark a node SPM unless a driver overrides it.
(MHP flag? Sorry David :P)

> > 
> > ~Gregory
> 
> Please let me know if you have further concerns or suggestions.
> 

I'll look at the patch details a bit more, but generally I like the
direction - with an obvious note that I have a biased given the above.

~Gregory