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Re: [PATCH v7 3/6] Kconfig: Make cpu hotplug configurable
- To: Mykyta Poturai <Mykyta_Poturai@xxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 30 Mar 2026 14:19:10 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 30 Mar 2026 12:19:17 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.
Jan
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