RE: [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.