linux-kernel - Re: [PATCH RESEND] x86/cpu/topology: remove limit of CPUs due to noapic on x86

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Message-ID: <87wmgjv7gz.ffs@tglx>
Date: Sun, 01 Dec 2024 14:49:32 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: "Fernando F. Mancera" <ffmancera@...eup.net>, Ingo Molnar
 <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH RESEND] x86/cpu/topology: remove limit of CPUs due to
 noapic on x86_64

On Mon, Nov 25 2024 at 16:13, Fernando F. Mancera wrote:
> On 25/11/2024 09:37, Ingo Molnar wrote:
>> So what's the motivation? Arguably the x86-64 boot option behavior was
>> weird: a working local APIC is very much needed to have an SMP system.
>> 
>
> Sorry if I am wrong here but I am not an expert on the matter. I 
> believed that "noapic" disables I/O APIC which handles external 
> interrupts while the local APICs are still enabled as they are managed 
> by "nolapic". If that is the case, SMP should still be possible.
>
> If both I/O APIC and Local APICs are disabled then the kernel should 
> fallback to a single CPU mode. That is the behavior that kernel has with 
> my patch.
>
> The motivation is to fix multiple users with systems that requires 
> "noapic" to work and after the rework their systems are using a single CPU.

Sorry, my bad. I messed up the "noapic" option handling.