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From: Thomas Gleixner <tglx@linutronix.de>
To: "Zhang, Rui" <rui.zhang@intel.com>, "linux-kernel@vger.kernel.org"
 <linux-kernel@vger.kernel.org>
Cc: "jgross@suse.com" <jgross@suse.com>, "mikelley@microsoft.com"
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 "Mehta, Sohil" <sohil.mehta@intel.com>, "andy@infradead.org"
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Subject: Re: [patch 37/53] x86/cpu: Detect real BSP on crash kernels
In-Reply-To: <7443f3200ef0be756566059e8c9b0657f3cefcff.camel@intel.com>
References: <20230807130108.853357011@linutronix.de>
 <20230807135028.381851690@linutronix.de>
 <7443f3200ef0be756566059e8c9b0657f3cefcff.camel@intel.com>
Date: Mon, 08 Jan 2024 15:54:04 +0100
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On Mon, Jan 08 2024 at 14:11, Zhang, Rui wrote:
>> +static __init void check_for_kdump_kernel(void)
>> +{
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0u32 bsp_apicid;
>> +
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0/*
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * There is no real good way =
to detect whether this a kdump()
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * kernel, but except on the =
Voyager SMP monstrosity which is
>> not
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * longer supported, the real=
 BSP has always the lowest
>> numbered
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * APIC ID. If a crash happen=
ed on an AP, which then ends up
>> as
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * boot CPU in the kdump() ke=
rnel, then sending INIT to the
>> real
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 * BSP would reset the whole =
system.
>> +=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 */
>
>
> Hi, Thomas,
>
> Unfortunately this causes a regression on Intel Meteorlake platform,
> where the BSP APIC ID is NOT the lowest numbered APIC ID (instead,
> CPU12, the first Ecore CPU, has APIC ID 0).

Bah. Let me think about that.