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RE: [PATCH v2 07/24] xen/device-tree: Parse 'cpu-map' node for CPU topology exploration
- To: Hirokazu Takahashi <taka@xxxxxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>
- From: Hirokazu Takahashi <taka@xxxxxxxxxxxxx>
- Date: Wed, 3 Jun 2026 13:01:04 +0000
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Wed, 03 Jun 2026 13:01:27 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-topic: [PATCH v2 07/24] xen/device-tree: Parse 'cpu-map' node for CPU topology exploration
Hello,
> > > +
> > > +#ifdef CONFIG_DT_CPU_TOPOLOGY
> > > +extern struct cpu_topology cpu_topology[NR_CPUS];
> >
> > If at all possible, no new NR_CPUS dimensioned arrays please.
>
> This is also part of the code imported from Linux kernel 7.0, which is used to
> store
> unique topology data per CPU.
>
> I kept it as an array to match the original Linux source, but I understand the
> concern
> about adding new NR_CPUS dimensioned arrays in Xen. I can refactor it to use
> Xen's
> per-CPU infrastructure instead.
Just a quick follow-up to my previous email:
While I was looking into moving the topology data to a per-CPU variable,
I remembered the ongoing effort by the team working on CPU hotplug.
This introduces a dilemma: during the early boot phase when we parse
the Device Tree topology, the per-CPU areas for CPUs that are not yet active
(or yet to be hotplugged) have not been allocated yet. Therefore, we wouldn't
have a place to record the topology information for those inactive CPUs
at boot time.
Should we have the hotplug team parse the Device Tree topology information
on-demand when a CPU is hot-plugged?
Currently, the functions used to read the topology from the Device Tree are
marked with '__init'. Removing '__init' to keep these functions resident in
memory feels like a waste of memory.
What do you think would be the best approach here?
Thank you,
Hirokazu Takahashi.
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