linux-kernel - Re: [PATCH v2] x86/apic: Do apic driver probe for "nosmp" use case

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Message-ID: <8d6789ba-85d3-3482-4bbb-3693073bea06@linux.intel.com>
Date:   Thu, 7 Apr 2022 20:54:56 -0700
From:   Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>
Cc:     linux-kernel@...r.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH v2] x86/apic: Do apic driver probe for "nosmp" use case



On 4/6/22 3:50 PM, Thomas Gleixner wrote:
> On Wed, Apr 06 2022 at 18:54, Kuppuswamy Sathyanarayanan wrote:
>> diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
>> index b70344bf6600..79b8b521981c 100644
>> --- a/arch/x86/kernel/apic/apic.c
>> +++ b/arch/x86/kernel/apic/apic.c
>> @@ -1419,22 +1419,22 @@ void __init apic_intr_mode_init(void)
>>   		return;
>>   	case APIC_VIRTUAL_WIRE:
>>   		pr_info("APIC: Switch to virtual wire mode setup\n");
>> -		default_setup_apic_routing();
>>   		break;
>>   	case APIC_VIRTUAL_WIRE_NO_CONFIG:
>>   		pr_info("APIC: Switch to virtual wire mode setup with no configuration\n");
>>   		upmode = true;
>> -		default_setup_apic_routing();
>>   		break;
>>   	case APIC_SYMMETRIC_IO:
>>   		pr_info("APIC: Switch to symmetric I/O mode setup\n");
>> -		default_setup_apic_routing();
>>   		break;
>>   	case APIC_SYMMETRIC_IO_NO_ROUTING:
>>   		pr_info("APIC: Switch to symmetric I/O mode setup in no SMP routine\n");
>> +		upmode = true;
> 
> Why? The changelog tells nothing about this change.
> 
> And it's not correct because the APIC configuration is there, otherwise
> __apic_intr_mode_select() would have returned APIC_VIRTUAL_WIRE_NO_CONFIG.

Makes sense. I initially thought since APIC_SYMMETRIC_IO_NO_ROUTING is
only used in non-smp case (setup_max_cpus == 0), we can force
uniprocessor mode. But checking again, my assumption is incorrect. It
will only be used in non MADT/MP case. I will remove the upmode change 
in next version.

> 
> Thanks,
> 
>          tglx

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer