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RE: [PATCH v3 1/3] xen/device-tree: Parse 'cpu-map' node for CPU topology exploration
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Hirokazu Takahashi <taka@xxxxxxxxxxxxx>
- Date: Thu, 2 Jul 2026 08:07:14 +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: Thu, 02 Jul 2026 08:07:48 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Thread-index: AQHdCBJguMZzJpb/JEe22XtErI4LcbZWqjWAgAHISeCAABufgIAAi7BQgAClhICAACFHIA==
- Thread-topic: [PATCH v3 1/3] xen/device-tree: Parse 'cpu-map' node for CPU topology exploration
Hello,
> >>>>> +#else /* CONFIG_CPU_TOPOLOGY */
> >>>>> +
> >>>>> +#define cpu_topology ((struct cpu_topology *)NULL)
> >>>>
> >>>> Why exactly is this needed? It very much looks as if its presence may
> >>>> hide
> >> bugs.
> >>>
> >>> In the current code, the condition if ( cpu_topology ) is used to cover
> >>> both
> >> cases:
> >>> when CONFIG_CPU_TOPOLOGY is disabled, and when memory allocation for
> >>> topology table fails.
> >>>
> >>> The next patch uses if ( cpu_topology ).
> >>>
> >>> static int setup_cpu_sibling_map(int cpu)
> >>> {
> >>> if ( !zalloc_cpumask_var(&per_cpu(cpu_sibling_mask, cpu)) ||
> >>> !zalloc_cpumask_var(&per_cpu(cpu_core_mask, cpu)) )
> >>> return -ENOMEM;
> >>>
> >>> if ( cpu_topology )
> >>
> >> if ( IS_ENABLED(CONFIG_GENERIC_CPU_TOPOLOGY) )
> >
> > This line should be if ( IS_ENABLED(CONFIG_GENERIC_CPU_TOPOLOGY) &&
> > cpu_topology )
> > because 'cpu_topology' can be NULL even if CONFIG_GENERIC_CPU_TOPOLOGY is
> > on.
> >
> > void __init init_cpu_topology(void)
> > {
> > unsigned int cpu;
> >
> > cpu_topology = xzalloc_array(struct cpu_topology, nr_cpu_ids);
> > if ( !cpu_topology )
> > {
> > printk(XENLOG_ERR "Failed to allocate memory for cpu_topology
> > table\n");
> > return;
> > }
>
> Ah yes. The odd #define can nevertheless go away, which is the important part.
Here is my current plan:
In the `cpu-topology.h` header, I will provide an initialization function for
`cpu_sibling_mask`
and `cpu_core_mask` when `CONFIG_GENERIC_CPU_TOPOLOGY` is on, along with another
definition when it is off, and then call them from `setup_cpu_sibling_map()`.
Thank you,
Hirokazu Takahashi.
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