linux-kernel - Re: [PATCH] Revert "x86/apic/x2apic: Implement IPI shorthands support"

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Date:   Tue, 20 Dec 2022 12:38:46 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        x86@...nel.org, kexec@...ts.infradead.org
Subject: Re: [PATCH] Revert "x86/apic/x2apic: Implement IPI shorthands
 support"

On Tue, Dec 20, 2022 at 01:34:58PM +0800, Baoquan He wrote:
> This reverts commit 43931d350f30c6cd8c2f498d54ef7d65750abc92.
> 
> On kvm guest with 4 cpus deployed, when adding 'nr_cpus=2' to normal
> kernel's cmdline, and triggering crash to jump to kdump kernel, kdump
> kernel will stably hang. Reverting commit 43931d350f30 ("x86/apic/x2apic:
> Implement IPI shorthands support") can fix it.
> 
> The problem will disappear if removing 'nr_cpus=2' from normal kerne's
> cmdline.

And the root cause for this is... ? Does the kvm x2apic emulation
somehow get upset when we shorthand CPUs that haven't been initialized?