linux-kernel - Re: Is this code chunk in init_apic

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Date:   Wed, 11 Mar 2020 20:39:37 +0800
From:   Cao jin <caoj.fnst@...fujitsu.com>
To:     Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
CC:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
Subject: Re: Is this code chunk in init_apic_mappings() superfluous?

On 3/7/20 7:27 PM, Thomas Gleixner wrote:
> Cao jin <caoj.fnst@...fujitsu.com> writes:
>> Hi,
>>
>> I am trying to figure the following code chunk out:
>>
>> 	if (x2apic_mode) {
>> 		boot_cpu_physical_apicid = read_apic_id();
>> 		return;
>> 	}
>>
>> As my understanding, even in x2APIC mode, boot_cpu_physical_apicid is
>> also got via early_acpi_process_madt --> ... --> register_lapic_address,
>> so, is it for any corner case I am not aware?
> 
> The case when there are neither ACPI nor valid mptables. Unlikely, but
> possible in theory.
> 

I noticed this case in its calling comments, but failed to associate it
with a x2APIC...

So init_apic_mapping() is for 2 corner cases that doesn't map APIC
register before: one you mentioned here, and MP default configuration
the other.

Thanks Thomas.

-- 
Sincerely,
Cao jin