[RFC PATCH 28/32] ACPI: add support to register CPUs based on the _STA enabled bit

[James Morse]

acpi_processor_get_info() registers all present CPUs. Registering a
CPU is what creates the sysfs entries and triggers the udev
notifications.

arm64 virtual machines that support 'virtual cpu hotplug' use the
enabled bit to indicate whether the CPU can be brought online, as
the existing ACPI tables require all hardware to be described and
present.

If firmware describes a CPU as present, but disabled, skip the
registration. Such CPUs are present, but can't be brought online for
whatever reason. (e.g. firmware/hypervisor policy).

Once firmware sets the enabled bit, the CPU can be registered and
brought online by user-space. Online CPUs, or CPUs that are missing
an _STA method must always be registered.

Signed-off-by: James Morse <james.morse at arm.com>
---
 drivers/acpi/acpi_processor.c | 34 +++++++++++++++++++++++++++++++++-
 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/drivers/acpi/acpi_processor.c b/drivers/acpi/acpi_processor.c
index 572a12672c0e..20e810a066da 100644
--- a/drivers/acpi/acpi_processor.c
+++ b/drivers/acpi/acpi_processor.c
@@ -194,6 +194,35 @@ static int acpi_processor_make_present(struct acpi_processor *pr)
 	return ret;
 }
 
+static int acpi_processor_make_enabled(struct acpi_processor *pr)
+{
+	unsigned long long sta;
+	acpi_status status;
+	bool present, enabled;
+
+	if (!acpi_has_method(pr->handle, "_STA"))
+		return arch_register_cpu(pr->id);
+
+	status = acpi_evaluate_integer(pr->handle, "_STA", NULL, &sta);
+	if (ACPI_FAILURE(status))
+		return -ENODEV;
+
+	present = sta & ACPI_STA_DEVICE_PRESENT;
+	enabled = sta & ACPI_STA_DEVICE_ENABLED;
+
+	if (cpu_online(pr->id) && (!present || !enabled)) {
+		pr_err_once(FW_BUG "CPU %u is online, but described as not present or disabled!\n", pr->id);
+		add_taint(TAINT_FIRMWARE_WORKAROUND, LOCKDEP_STILL_OK);
+	} else if (!present || !enabled) {
+		return -ENODEV;
+	}
+
+	if (get_cpu_device(pr->id))
+		return -EEXIST;
+
+	return arch_register_cpu(pr->id);
+}
+
 static int acpi_processor_get_info(struct acpi_device *device)
 {
 	union acpi_object object = { 0 };
@@ -271,7 +300,7 @@ static int acpi_processor_get_info(struct acpi_device *device)
 	}
 
 	if (!invalid_logical_cpuid(pr->id) && cpu_present(pr->id)) {
-		int ret = arch_register_cpu(pr->id);
+		int ret = acpi_processor_make_enabled(pr);
 		if (ret)
 			return ret;
 	}
@@ -478,6 +507,9 @@ static void acpi_processor_post_eject(struct acpi_device *device)
 		acpi_processor_make_not_present(device);
 		return;
 	}
+
+	if (cpu_present(pr->id) && !(sta & ACPI_STA_DEVICE_ENABLED))
+		arch_unregister_cpu(pr->id);
 }
 
 #ifdef CONFIG_X86
-- 
2.30.2