Re: [PATCHv6 24/30] x86/topology: Disable CPU online/offline control for TDX guests

[Thomas Gleixner]

On Wed, Mar 16 2022 at 05:08, Kirill A. Shutemov wrote:
> From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
>
> Unlike regular VMs, TDX guests use the firmware hand-off wakeup method
> to wake up the APs during the boot process. This wakeup model uses a
> mailbox to communicate with firmware to bring up the APs. As per the
> design, this mailbox can only be used once for the given AP, which means
> after the APs are booted, the same mailbox cannot be used to
> offline/online the given AP. More details about this requirement can be
> found in Intel TDX Virtual Firmware Design Guide, sec titled "AP
> initialization in OS" and in sec titled "Hotplug Device".
>
> Since the architecture does not support any method of offlining the
> CPUs, disable CPU hotplug support in the kernel.
>
> Since this hotplug disable feature can be re-used by other VM guests,
> add a new CC attribute CC_ATTR_HOTPLUG_DISABLED and use it to disable
> the hotplug support.
>
> With hotplug disabled, /sys/devices/system/cpu/cpuX/online sysfs option
> will not exist for TDX guests.

I can't find any code which removes the online file. The write will fail
with -EOPNOTSUPP, right?

> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

Other than that:

Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>