x86/topology: Disable CPU online/offline control for TDX guests

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.

Attempt to offline CPU will fail with -EOPNOTSUPP.

Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20220405232939.73860-25-kirill.shutemov@linux.intel.com

include/linux/cc_platform.h

@@ -72,6 +72,16 @@ enum cc_attr {
 	 * Examples include TDX guest & SEV.
 	 */
 	CC_ATTR_GUEST_UNROLL_STRING_IO,
+
+	/**
+	 * @CC_ATTR_HOTPLUG_DISABLED: Hotplug is not supported or disabled.
+	 *
+	 * The platform/OS is running as a guest/virtual machine does not
+	 * support CPU hotplug feature.
+	 *
+	 * Examples include TDX Guest.
+	 */
+	CC_ATTR_HOTPLUG_DISABLED,
 };
 
 #ifdef CONFIG_ARCH_HAS_CC_PLATFORM