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

diff --git a/arch/x86/coco/core.c b/arch/x86/coco/core.c
index df08edc94f9bab9807a5bfc796d04821e5bfaab9..70956f9d7c7e5244ffe2a0fe3be9512aea0b6edd 100644 (file)
--- a/arch/x86/coco/core.c
+++ b/arch/x86/coco/core.c
@@ -20,6 +20,7 @@ static bool intel_cc_platform_has(enum cc_attr attr)
 {
        switch (attr) {
        case CC_ATTR_GUEST_UNROLL_STRING_IO:
+       case CC_ATTR_HOTPLUG_DISABLED:
                return true;
        default:
                return false;

diff --git a/include/linux/cc_platform.h b/include/linux/cc_platform.h
index efd8205282da74e12c4cb3e1fcb99d899f8fed64..691494bbaf5af4c174c599e0278a43a0c3658bfd 100644 (file)
--- a/include/linux/cc_platform.h
+++ b/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

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 5797c2a7a93f415dd622c8aaf34bb98e9e7d7e61..edb8c199f6a37946fc6c7f25a66632a72cd872d9 100644 (file)
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -35,6 +35,7 @@
 #include <linux/percpu-rwsem.h>
 #include <linux/cpuset.h>
 #include <linux/random.h>
+#include <linux/cc_platform.h>
 
 #include <trace/events/power.h>
 #define CREATE_TRACE_POINTS
@@ -1186,6 +1187,12 @@ out:
 
 static int cpu_down_maps_locked(unsigned int cpu, enum cpuhp_state target)
 {
+       /*
+        * If the platform does not support hotplug, report it explicitly to
+        * differentiate it from a transient offlining failure.
+        */
+       if (cc_platform_has(CC_ATTR_HOTPLUG_DISABLED))
+               return -EOPNOTSUPP;
        if (cpu_hotplug_disabled)
                return -EBUSY;
        return _cpu_down(cpu, 0, target);