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

[Oleksandr Tyshchenko <olekstysh@xxxxxxxxx>]

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@xxxxxxxx>
> > > 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