linux-kernel - Re: [PATCH] x86/srat: Conditional apic_id vs MAX_LOCAL

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Message-ID: <Z8GCyyt0Dud0kvwq@gmail.com>
Date: Fri, 28 Feb 2025 10:32:59 +0100
From: Ingo Molnar <mingo@...nel.org>
To: WangYuli <wangyuli@...ontech.com>
Cc: dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, x86@...nel.org,
	hpa@...or.com, linux-kernel@...r.kernel.org, zhanjun@...ontech.com,
	niecheng1@...ontech.com, chenlinxuan@...ontech.com,
	Huacai Chen <chenhuacai@...ngson.cn>,
	Wentao Guan <guanwentao@...ontech.com>
Subject: Re: [PATCH] x86/srat: Conditional apic_id vs MAX_LOCAL_APIC
 comparison


* WangYuli <wangyuli@...ontech.com> wrote:

> The apic_id employed within the acpi_numa_processor_affinity_init()
> function is derived from the acpi_srat_cpu_affinity structure defined
> in acpi/actbl3.h.
> 
> When CONFIG_X86_X2APIC is not enabled, its maximum value is limited
> to 255.

How does the compiler know the value is limited to 0-255?

<acpi/actbl3.h> has an u32 apic_id field:

 struct acpi_srat_x2apic_cpu_affinity {
        struct acpi_subtable_header header;
        u16 reserved;           /* Reserved, must be zero */
        u32 proximity_domain;
        u32 apic_id;
        ^^^^^^^^^^^^
        u32 flags;
        u32 clock_domain;
        u32 reserved2;
 };

and the apic_id local variable in acpi_numa_processor_affinity_init() 
is an 'int':

        int apic_id;

Neither is limited to 0-255.

Thanks,

	Ingo