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Date:   Mon, 17 Apr 2023 12:30:50 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
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Subject: Re: [patch 00/37] cpu/hotplug, x86: Reworked parallel CPU bringup
Message-ID: <20230417103050.GF83892@hirez.programming.kicks-ass.net>
References: <20230414225551.858160935@linutronix.de>
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On Sat, Apr 15, 2023 at 01:44:13AM +0200, Thomas Gleixner wrote:

> Background
> ----------
> 
> The reason why people are interested in parallel bringup is to shorten
> the (kexec) reboot time of cloud servers to reduce the downtime of the
> VM tenants. There are obviously other interesting use cases for this
> like VM startup time, embedded devices...

...

>   There are two issue there:
> 
>     a) The death by MCE broadcast problem
> 
>        Quite some (contemporary) x86 CPU generations are affected by
>        this:
> 
>          - MCE can be broadcasted to all CPUs and not only issued locally
>            to the CPU which triggered it.
> 
>          - Any CPU which has CR4.MCE == 0, even if it sits in a wait
>            for INIT/SIPI state, will cause an immediate shutdown of the
>            machine if a broadcasted MCE is delivered.

When doing kexec, CR4.MCE should already have been set to 1 by the prior
kernel, no?