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

[Valentin Schneider <valentin.schneider@arm.com>]

On 22/11/21 16:47, Sebastian Andrzej Siewior wrote:
> From: "Longpeng(Mike)" <longpeng2@huawei.com>
>
> 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.
>

For my own education, this is talking about *host* CPU hotplug, right?

> 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@huawei.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Link: https://lore.kernel.org/r/20210901051143.2752-1-longpeng2@huawei.com
> ---
>
> 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.

Virt stuff notwithstanding, that looks OK to me.
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>

That said, AFAICT only powerpc makes actual use of the state being set to
CPU_UP_PREPARE, it looks to be needless bookkeeping for everyone else (and
there's archs that seem happy using only CPU_DEAD / CPU_POST_DEAD).

>
>  kernel/smpboot.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/kernel/smpboot.c b/kernel/smpboot.c
> index f6bc0bc8a2aab..34958d7fe2c1c 100644
> --- a/kernel/smpboot.c
> +++ b/kernel/smpboot.c
> @@ -392,6 +392,13 @@ int cpu_check_up_prepare(int cpu)
>                */
>               return -EAGAIN;
>
> +	case CPU_UP_PREPARE:
> +		/*
> +		 * Timeout while waiting for the CPU to show up. Allow to try
> +		 * again later.
> +		 */
> +		return 0;
> +
>       default:
>
>               /* Should not happen.  Famous last words. */
> --
> 2.33.1