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

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Message-ID: <87ttbnv75j.ffs@tglx>
Date: Sun, 01 Dec 2024 14:56:24 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Fernando Fernandez Mancera <ffmancera@...eup.net>,
 linux-kernel@...r.kernel.org
Cc: x86@...nel.org, Fernando Fernandez Mancera <ffmancera@...eup.net>
Subject: Re: [PATCH RESEND] x86/cpu/topology: remove limit of CPUs due to
 noapic on x86_64

On Sun, Nov 24 2024 at 18:45, Fernando Fernandez Mancera wrote:
> On x86_64 the command line parameter "noapic" should not limit the
> number of possible CPUs, as it only limits the use of IRQ sharing or
> device IRQ remapping. Only on x86_32 the command line parameter
> "nolapic" limits the number of possible CPUs to one. This restores the
> behavior previous to the rework of possible CPU management.
>
> Fixes: 7c0edad3643f ("x86/cpu/topology: Rework possible CPU management")
> Signed-off-by: Fernando Fernandez Mancera <ffmancera@...eup.net>
> ---
> RESEND: original patch https://lkml.org/lkml/2024/9/7/160

Please use https://lore.kernel.org/all/$MSGID references.

> ---
>  arch/x86/kernel/cpu/topology.c | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/topology.c b/arch/x86/kernel/cpu/topology.c
> index 621a151ccf7d..5f10a010e35a 100644
> --- a/arch/x86/kernel/cpu/topology.c
> +++ b/arch/x86/kernel/cpu/topology.c
> @@ -428,10 +428,16 @@ void __init topology_apply_cmdline_limits_early(void)
>  {
>  	unsigned int possible = nr_cpu_ids;
>  
> -	/* 'maxcpus=0' 'nosmp' 'nolapic' 'disableapic' 'noapic' */
> -	if (!setup_max_cpus || ioapic_is_disabled || apic_is_disabled)
> +	/* 'maxcpus=0' 'nosmp' */
> +	if (!setup_max_cpus)

That's wrong. If the local APIC is disabled, SMP is not possible.

>  		possible = 1;
>  
> +#if defined(CONFIG_X86_32)
> +	/* 'nolapic' 'disableapic' 'noapic' */
> +	if (apic_is_disabled || ioapic_is_disabled)

I double checked the original behaviour. A disabled IOAPIC did not
prevent SMP on 32bit either.

> +		possible = 1;
> +#endif

So the condition wants to be:

	if (!setup_max_cpus || apic_is_disabled)


>  	/* 'possible_cpus=N' */
>  	possible = min_t(unsigned int, max_possible_cpus, possible);
>  
> @@ -443,8 +449,14 @@ void __init topology_apply_cmdline_limits_early(void)
>  
>  static __init bool restrict_to_up(void)
>  {
> -	if (!smp_found_config || ioapic_is_disabled)
> +	if (!smp_found_config)
>  		return true;
> +
> +#if defined(CONFIG_X86_32)
> +	if (ioapic_is_disabled)
> +		return true;
> +#endif

That wants to go away too.

Thanks,

        tglx