Re: [patch 00/37] cpu/hotplug, x86: Reworked parallel CPU bringup

Peter Zijlstra
2023-04-17T10:33:56Z

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?