[PATCH RFC 1/4] arm64: kernel: Handle disabled[(+)present] cpus in MADT/GICC during init

Salil Mehta salil.mehta at huawei.com
Thu Jun 25 09:37:54 EDT 2020

With ACPI enabled, cpus get identified by the presence of the GICC
entry in the MADT Table. Each GICC entry part of MADT presents cpu as
enabled or disabled. As of now, the disabled cpus are skipped as
physical cpu hotplug is not supported. These remain disabled even after
the kernel has booted.

To support virtual cpu hotplug(in which case disabled vcpus could be
hotplugged even after kernel has booted), QEMU will populate MADT Table
with appropriate details of GICC entry for each possible(present+disabled)
vcpu. Now, during the init time vcpus will be identified as present or
disabled. To achieve this, below changes have been made with respect to
the present/possible vcpu handling along with the mentioned reasoning:

1. Identify all possible(present+disabled) vcpus at boot/init time
   and set their present mask and possible mask. In the existing code,
   cpus are being marked present quite late within smp_prepare_cpus()
   function, which gets called in context to the kernel thread. Since
   the cpu hotplug is not supported, present cpus are always equal to
   the possible cpus. But with cpu hotplug enabled, this assumption is
   not true. Hence, present cpus should be marked while MADT GICC entries
   are bring parsed for each vcpu.
2. Set possible cpus to include disabled. This needs to be done now
   while parsing MADT GICC entries corresponding to each vcpu as the
   disabled vcpu info is available only at this point as for hotplug
   case possible vcpus is not equal to present vcpus.
3. We will store the parsed madt/gicc entry even for the disabled vcpus
   during init time. This is needed as some modules like PMU registers
   IRQs for each possible vcpus during init time. Therefore, a valid
   entry of the MADT GICC should be present for all possible vcpus.
4. Refactoring related to DT/OF is also done to align it with the init
   changes to support vcpu hotplug.

Signed-off-by: Salil Mehta <salil.mehta at huawei.com>
Signed-off-by: Xiongfeng Wang <wangxiongfeng2 at huawei.com>
 arch/arm64/kernel/smp.c | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index e43a8ff19f0f..51a707928302 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -509,13 +509,12 @@ static int __init smp_cpu_setup(int cpu)
 	if (ops->cpu_init(cpu))
 		return -ENODEV;
-	set_cpu_possible(cpu, true);
 	return 0;
 static bool bootcpu_valid __initdata;
 static unsigned int cpu_count = 1;
+static unsigned int disabled_cpu_count;
 static struct acpi_madt_generic_interrupt cpu_madt_gicc[NR_CPUS];
@@ -534,10 +533,17 @@ struct acpi_madt_generic_interrupt *acpi_cpu_get_madt_gicc(int cpu)
 static void __init
 acpi_map_gic_cpu_interface(struct acpi_madt_generic_interrupt *processor)
+	unsigned int total_cpu_count = disabled_cpu_count + cpu_count;
 	u64 hwid = processor->arm_mpidr;
 	if (!(processor->flags & ACPI_MADT_ENABLED)) {
 		pr_debug("skipping disabled CPU entry with 0x%llx MPIDR\n", hwid);
+		cpu_madt_gicc[total_cpu_count] = *processor;
+		set_cpu_possible(total_cpu_count, true);
+		disabled_cpu_count++;
@@ -546,7 +552,7 @@ acpi_map_gic_cpu_interface(struct acpi_madt_generic_interrupt *processor)
-	if (is_mpidr_duplicate(cpu_count, hwid)) {
+	if (is_mpidr_duplicate(total_cpu_count, hwid)) {
 		pr_err("duplicate CPU MPIDR 0x%llx in MADT\n", hwid);
@@ -567,9 +573,9 @@ acpi_map_gic_cpu_interface(struct acpi_madt_generic_interrupt *processor)
 	/* map the logical cpu id to cpu MPIDR */
-	cpu_logical_map(cpu_count) = hwid;
+	cpu_logical_map(total_cpu_count) = hwid;
-	cpu_madt_gicc[cpu_count] = *processor;
+	cpu_madt_gicc[total_cpu_count] = *processor;
 	 * Set-up the ACPI parking protocol cpu entries
@@ -580,8 +586,10 @@ acpi_map_gic_cpu_interface(struct acpi_madt_generic_interrupt *processor)
 	 * initialize the cpu if the parking protocol is
 	 * the only available enable method).
-	acpi_set_mailbox_entry(cpu_count, processor);
+	acpi_set_mailbox_entry(total_cpu_count, processor);
+	set_cpu_possible(total_cpu_count, true);
+	set_cpu_present(total_cpu_count, true);
@@ -614,6 +622,9 @@ static void __init acpi_parse_and_init_cpus(void)
 				      acpi_parse_gic_cpu_interface, 0);
+	pr_debug("possible cpus(%u) present cpus(%u) disabled cpus(%u)\n",
+		 cpu_count+disabled_cpu_count, cpu_count, disabled_cpu_count);
 	 * In ACPI, SMP and CPU NUMA information is provided in separate
 	 * static tables, namely the MADT and the SRAT.
@@ -684,6 +695,9 @@ static void __init of_parse_and_init_cpus(void)
 		cpu_logical_map(cpu_count) = hwid;
 		early_map_cpu_to_node(cpu_count, of_node_to_nid(dn));
+		set_cpu_possible(cpu_count, true);
+		set_cpu_present(cpu_count, true);
@@ -768,7 +782,6 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
 		if (err)
-		set_cpu_present(cpu, true);

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