Re: [PATCH v7 3/6] Kconfig: Make cpu hotplug configurable

[Oleksandr Tyshchenko <olekstysh@gmail.com>]

On 4/8/26 15:27, Jan Beulich wrote:
> On 08.04.2026 14:21, Mykyta Poturai wrote:

Hello all.

>> On 3/30/26 15:19, Jan Beulich wrote:
>>> On 30.03.2026 13:59, Mykyta Poturai wrote:
>>>> For the purposes of certification, we want as little code as possible to
>>>> be unconditionally compiled in. Make CPU hotplug and SMT operations
>>>> configurable to ease the process. This will also help with introducing
>>>> CPU hotplug on Arm, where it needs to be configurable.
>>>>
>>>> Signed-off-by: Mykyta Poturai <mykyta_poturai@epam.com>
>>>
>>> Looks largely okay from a technical pov; one nit and one (repeated) remark
>>> below.
>>>
>>>> --- a/xen/arch/x86/sysctl.c
>>>> +++ b/xen/arch/x86/sysctl.c
>>>> @@ -53,6 +53,11 @@ static long cf_check smt_up_down_helper(void *data)
>>>>        unsigned int cpu, sibling_mask = boot_cpu_data.x86_num_siblings - 1;
>>>>        int ret = 0;
>>>>    
>>>> +    if ( !IS_ENABLED(CONFIG_CPU_HOTPLUG) )
>>>> +    {
>>>> +        ASSERT_UNREACHABLE();
>>>> +        return -EOPNOTSUPP;
>>>> +    }
>>>>        opt_smt = up;
>>>
>>> Another blank line above this one perhaps?
>>>
>>>> --- a/xen/common/Kconfig
>>>> +++ b/xen/common/Kconfig
>>>> @@ -637,6 +637,14 @@ config SYSTEM_SUSPEND
>>>>    
>>>>    	  If unsure, say N.
>>>>    
>>>> +config CPU_HOTPLUG
>>>> +	bool "CPU online/offline support"
>>>> +	depends on X86
>>>> +	default y
>>>> +	help
>>>> +	  Enable support for bringing CPUs online and offline at runtime. On
>>>> +	  X86 this is required for disabling SMT.
>>>
>>> The name of this option may need input from others; I'm not quite convinced
>>> that this is a good name, as there's no true "hot-plugging" involved here.
>>> IOW I fear the present name is misleading.
>>
>> My first idea was "CONFIG_RUNTIME_CPU_CONTROL" I can switch back to it.
> 
> I could live with that, for at least not being misleading. CPU_ONLINE or
> CPU_ONLINE_OFFLINE might be another option, possibly better suited to
> later become a dependency (select) of a true CPU_HOTPLUG. As said, input
> from others may be helpful.

To me, CONFIG_RUNTIME_CPU_CONTROL sounds a little bit vague. Although we 
are indeed controlling CPUs at runtime, "control" could also mean 
cpufreq, power management, affinity pinning. I think 
CONFIG_CPU_ONLINE_OFFLINE is more precise, as it is clear from the name 
that we are transitioning CPUs between online and offline states.


> 
> Jan
>