Re: [PATCH] cpu/hotplug: Allow the CPU in CPU_UP_PREPARE state to be brought up again.

[Thomas Gleixner <tglx@xxxxxxxxxxxxx>]

On Mon, Nov 22 2021 at 16:47, Sebastian Andrzej Siewior wrote:
> From: "Longpeng(Mike)" <longpeng2@xxxxxxxxxx>
>
> A CPU will not show up in virtualized environment which includes an
> Enclave. The VM splits its resources into a primary VM and a Enclave
> VM. While the Enclave is active, the hypervisor will ignore all requests
> to bring up a CPU and this CPU will remain in CPU_UP_PREPARE state.
> The kernel will wait up to ten seconds for CPU to show up
> (do_boot_cpu()) and then rollback the hotplug state back to
> CPUHP_OFFLINE leaving the CPU state in CPU_UP_PREPARE. The CPU state is
> set back to CPUHP_TEARDOWN_CPU during the CPU_POST_DEAD stage.
>
> After the Enclave VM terminates, the primary VM can bring up the CPU
> again.
>
> Allow to bring up the CPU if it is in the CPU_UP_PREPARE state.
>
> [bigeasy: Rewrite commit description.]
>
> Signed-off-by: Longpeng(Mike) <longpeng2@xxxxxxxxxx>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/r/20210901051143.2752-1-longpeng2@xxxxxxxxxx
> ---
>
> For XEN: this changes the behaviour as it allows to invoke
> cpu_initialize_context() again should it have have earlier. I *think*
> this is okay and would to bring up the CPU again should the memory
> allocation in cpu_initialize_context() fail.

Any comment from XEN folks?

Thanks,

        tglx