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Re: [PATCH v2 6/6] x86/irq: do not set nr_irqs based on nr_irqs_gsi in APIC mode
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 5 Jul 2022 16:22:11 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 05 Jul 2022 14:22:26 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 30.06.2022 10:54, Roger Pau Monne wrote:
> When using an APIC do not set nr_irqs based on a factor of nr_irqs_gsi
> (currently x8), and instead do so exclusively based on the amount of
> available vectors on the system.
>
> There's no point in setting nr_irqs to a value higher than the
> available set of vectors, as vector allocation will fail anyway.
Only if in the earlier calculation you include hotplug CPUs.
> --- a/xen/arch/x86/irq.c
> +++ b/xen/arch/x86/irq.c
> @@ -426,8 +426,7 @@ int __init init_irq_data(void)
> (x2apic_enabled && !x2apic_phys) ? x2apic_max_cluster_id + 1
> : num_present_cpus();
>
> - nr_irqs = cpu_has_apic ? max(vec_spaces * NR_DYNAMIC_VECTORS,
> - 8 * nr_irqs_gsi)
> + nr_irqs = cpu_has_apic ? vec_spaces * NR_DYNAMIC_VECTORS
> : nr_irqs_gsi;
I wonder how useful it is to retain the !cpu_has_apic case here. I
don't have much hope for Xen to work properly if APIC use was
suppressed, and I'm unaware of x86-64 CPUs without APIC.
Jan
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