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

Kirill A. Shutemov
2022-03-17T20:47:49Z

On Thu, Mar 17, 2022 at 01:50:08PM +0100, Thomas Gleixner wrote:
> On Wed, Mar 16 2022 at 05:08, Kirill A. Shutemov wrote:
> > From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
> >
> > 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?

Right. Will fix the commit message.

--
Kirill A. Shutemov