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

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Message-ID: <Y7WYw0YH27hdsD11@work-vm>
Date:   Wed, 4 Jan 2023 15:18:27 +0000
From:   "Dr. David Alan Gilbert" <dgilbert@...hat.com>
To:     Baoquan He <bhe@...hat.com>, pbonzini@...hat.com,
        mlevitsk@...hat.com
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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"

* Baoquan He (bhe@...hat.com) wrote:
> On 12/20/22 at 12:38pm, Peter Zijlstra wrote:
> > 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?
> 
> Thanks for checking.
> 
> I haven't figure out the root cause. I haven't read the apic code for
> long time, and not familiar with the kvm code. So raise the issue to
> upstream.
> 
> I can do testing if any suggestion.
> 
> Add our virt dev Dr. David Alan Gilbert to CC.

Hmm I don't know that code well enough; cc'ing Paolo and Maxim.

Dave
-- 
Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK