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

[Peter Zijlstra]

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?