[PATCH v5 03/22] cc_platform: Add new attribute to prevent ACPI memory hotplug

[Kai Huang]

Platforms with confidential computing technology may not support ACPI
memory hotplug when such technology is enabled by the BIOS.  Examples
include Intel platforms which support Intel Trust Domain Extensions
(TDX).

If the kernel ever receives ACPI memory hotplug event, it is likely a
BIOS bug.  For ACPI memory hot-add, the kernel should speak out this is
a BIOS bug and reject the new memory.  For hot-removal, for simplicity
just assume the kernel cannot continue to work normally, and just BUG().

Add a new attribute CC_ATTR_ACPI_MEMORY_HOTPLUG_DISABLED to indicate the
platform doesn't support ACPI memory hotplug, so that kernel can handle
ACPI memory hotplug events for such platform.

In acpi_memory_device_{add|remove}(), add early check against this
attribute and handle accordingly if it is set.

Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
---
 drivers/acpi/acpi_memhotplug.c | 23 +++++++++++++++++++++++
 include/linux/cc_platform.h    | 10 ++++++++++
 2 files changed, 33 insertions(+)

diff --git a/drivers/acpi/acpi_memhotplug.c b/drivers/acpi/acpi_memhotplug.c
index 24f662d8bd39..94d6354ea453 100644
--- a/drivers/acpi/acpi_memhotplug.c
+++ b/drivers/acpi/acpi_memhotplug.c
@@ -15,6 +15,7 @@
 #include <linux/acpi.h>
 #include <linux/memory.h>
 #include <linux/memory_hotplug.h>
+#include <linux/cc_platform.h>
 
 #include "internal.h"
 
@@ -291,6 +292,17 @@ static int acpi_memory_device_add(struct acpi_device *device,
 	if (!device)
 		return -EINVAL;
 
+	/*
+	 * If the confidential computing platform doesn't support ACPI
+	 * memory hotplug, the BIOS should never deliver such event to
+	 * the kernel.  Report ACPI CPU hot-add as a BIOS bug and ignore
+	 * the memory device.
+	 */
+	if (cc_platform_has(CC_ATTR_ACPI_MEMORY_HOTPLUG_DISABLED)) {
+		dev_err(&device->dev, "[BIOS bug]: Platform doesn't support ACPI memory hotplug. New memory device ignored.\n");
+		return -EINVAL;
+	}
+
 	mem_device = kzalloc(sizeof(struct acpi_memory_device), GFP_KERNEL);
 	if (!mem_device)
 		return -ENOMEM;
@@ -334,6 +346,17 @@ static void acpi_memory_device_remove(struct acpi_device *device)
 	if (!device || !acpi_driver_data(device))
 		return;
 
+	/*
+	 * The confidential computing platform is broken if ACPI memory
+	 * hot-removal isn't supported but it happened anyway.  Assume
+	 * it is not guaranteed that the kernel can continue to work
+	 * normally.  Just BUG().
+	 */
+	if (cc_platform_has(CC_ATTR_ACPI_CPU_HOTPLUG_DISABLED)) {
+		dev_err(&device->dev, "Platform doesn't support ACPI memory hotplug. BUG().\n");
+		BUG();
+	}
+
 	mem_device = acpi_driver_data(device);
 	acpi_memory_remove_memory(mem_device);
 	acpi_memory_device_free(mem_device);
diff --git a/include/linux/cc_platform.h b/include/linux/cc_platform.h
index 9ce9256facc8..b831c24bd7f6 100644
--- a/include/linux/cc_platform.h
+++ b/include/linux/cc_platform.h
@@ -93,6 +93,16 @@ enum cc_attr {
 	 * Examples include TDX platform.
 	 */
 	CC_ATTR_ACPI_CPU_HOTPLUG_DISABLED,
+
+	/**
+	 * @CC_ATTR_ACPI_MEMORY_HOTPLUG_DISABLED: ACPI memory hotplug is
+	 *					  not supported.
+	 *
+	 * The platform/os does not support ACPI memory hotplug.
+	 *
+	 * Examples include TDX platform.
+	 */
+	CC_ATTR_ACPI_MEMORY_HOTPLUG_DISABLED,
 };
 
 #ifdef CONFIG_ARCH_HAS_CC_PLATFORM
-- 
2.36.1