linux-kernel - Re: Q. about KVM and CPU hotplug

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Date:   Tue, 30 Nov 2021 17:27:02 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        "Tian, Kevin" <kevin.tian@...el.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "Huang, Kai" <kai.huang@...el.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Gao, Chao" <chao.gao@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: Q. about KVM and CPU hotplug

On 11/30/21 15:05, Thomas Gleixner wrote:
> Why is this hotplug callback in the CPU starting section to begin with?

Just because the old notifier implementation used CPU_STARTING - in fact 
the commit messages say that CPU_STARTING was added partly *for* KVM 
(commit e545a6140b69, "kernel/cpu.c: create a CPU_STARTING cpu_chain 
notifier", 2008-09-08).

> If you stick it into the online section which runs on the hotplugged CPU
> in thread context:
> 
> 	CPUHP_AP_ONLINE_IDLE,
> 
> -->   	CPUHP_AP_KVM_STARTING,
> 
> 	CPUHP_AP_SCHED_WAIT_EMPTY,
> 
> then it is allowed to fail and it still works in the right way.

Yes, moving it to the online section should be fine; it wouldn't solve 
the TDX problem however.  Failure would rollback the hotplug and forbid 
hotplug altogether when TDX is loaded, which is not acceptable.

Paolo

> When onlining a CPU then there cannot be any vCPU task run on the
> CPU at that point.
> 
> When offlining a CPU then it's guaranteed that all user tasks and
> non-pinned kernel tasks have left the CPU, i.e. there cannot be a vCPU
> task around either.