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RE: [PATCH v6 1/5] xen/device-tree: Parse 'cpu-map' node for CPU topology exploration
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Hirokazu Takahashi <taka@xxxxxxxxxxxxx>
- Date: Sun, 19 Jul 2026 01:17:52 +0000
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- Cc: "Mykyta_Poturai@xxxxxxxx" <Mykyta_Poturai@xxxxxxxx>, 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: Sun, 19 Jul 2026 01:18:09 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Thread-index: AQHdE33lRSerj/u3kU6sfibELhxcvrZwJ2MAgAPosBA=
- Thread-topic: [PATCH v6 1/5] xen/device-tree: Parse 'cpu-map' node for CPU topology exploration
Hi Jan,
> > +/* SPDX-License-Identifier: GPL-2.0-only */
> > +
> > +#ifndef XEN_CPU_TOPOLOGY_H
> > +#define XEN_CPU_TOPOLOGY_H
> > +
> > +#include <xen/cpumask.h>
> > +
> > +#ifdef CONFIG_GENERIC_CPU_TOPOLOGY
> > +
> > +struct cpu_topology {
> > + cpumask_var_t thread_sibling;
>
> Btw, as this aspect becomes more apparent when looking at patch 2: What is the
> relationship of this (i.e. cpu_topology[cpu].thread_sibling) to
> per_cpu(cpu_sibling_mask, cpu)? The two ought to be identical, but I can't see
> this being guaranteed. And if they were identical, why would both be needed?
>
> > + cpumask_var_t core_sibling;
>
> Same for this vs the per-CPU cpu_core_mask.
I have considered the same thing before. However, considering the CPU hotplug
support that Mykyta Poturai is currently implementing, it is difficult to remove
thread_sibling and core_sibling from struct cpu_topology. We intend to use
cpu_topology to hold information for CPUs that haven't been hotplugged yet.
While it might make more sense to remove the per-CPU masks instead,
I hesitated to do so because the impact on x86 Xen would be quite large.
Thank you,
Hirokazu Takahashi.
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