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Re: [PATCH v3 1/1] xen/arm: fix sparse cpu_possible_map calculation on SMP boot
- To: Hirokazu Takahashi <taka@xxxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: "Orzel, Michal" <michal.orzel@xxxxxxx>
- Date: Thu, 2 Jul 2026 09:13:03 +0200
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>
- Delivery-date: Thu, 02 Jul 2026 07:13:17 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 02-Jul-26 00:40, Hirokazu Takahashi wrote:
> Currently, during ARM Xen's SMP initialization, if there is
> a Device Tree error (such as an invalid 'enable-method'),
> cpu_possible_map can end up being sparse.
>
> The issue here is that nr_cpu_ids is calculated in a way that
> doesn't properly account for the maximum CPU ID when the map is
> sparse, causing a mismatch. For example, if cpu_possible_map is
> 0xff0f, nr_cpu_ids becomes 12, but the actual maximum CPU ID
> is 15. Xen's common code is built on the assumption that
> 'CPU ID < nr_cpu_ids', so this mismatch can break things.
>
> To fix this, modify dt_smp_init_cpus() so that if the
> arch_cpu_init() call fails, we don't consume the CPU ID slot.
>
> Fixes: 4557c2292854 ("xen: arm: rewrite start of day page table and cpu bring
> up")
> Signed-off-by: Hirokazu Takahashi <taka@xxxxxxxxxxxxx>
Reviewed-by: Michal Orzel <michal.orzel@xxxxxxx>
~Michal
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