Reflect the QOM vCPUs ACPI CPU hotplug states in the `_STA.Present` and
and `_STA.Enabled` bits when the guest kernel evaluates the ACPI
`_STA` method during initialization, as well as when vCPUs are
hot-plugged or hot-unplugged. If the CPU is present then the its
`enabled` status can be fetched using architecture-specific code [1].

Reference:
[1] Example implementation of architecture-specific hook to fetch CPU
    `enabled status
    Link: https://github.com/salil-mehta/qemu/commit/c0b416b11e5af6505e558866f0eb6c9f3709173e

Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
---
 hw/acpi/cpu.c         | 38 ++++++++++++++++++++++++++++++++++----
 include/hw/core/cpu.h |  1 +
 2 files changed, 35 insertions(+), 4 deletions(-)

diff --git a/hw/acpi/cpu.c b/hw/acpi/cpu.c
index 9b03b4292e..23443f09a5 100644
--- a/hw/acpi/cpu.c
+++ b/hw/acpi/cpu.c
@@ -50,6 +50,18 @@ void acpi_cpu_ospm_status(CPUHotplugState *cpu_st, ACPIOSTInfoList ***list)
     }
 }
 
+static bool check_cpu_enabled_status(DeviceState *dev)
+{
+    CPUClass *k = dev ? CPU_GET_CLASS(dev) : NULL;
+    CPUState *cpu = CPU(dev);
+
+    if (cpu && (!k->cpu_enabled_status || k->cpu_enabled_status(cpu))) {
+        return true;
+    }
+
+    return false;
+}
+
 static uint64_t cpu_hotplug_rd(void *opaque, hwaddr addr, unsigned size)
 {
     uint64_t val = 0;
@@ -63,10 +75,11 @@ static uint64_t cpu_hotplug_rd(void *opaque, hwaddr addr, unsigned size)
     cdev = &cpu_st->devs[cpu_st->selector];
     switch (addr) {
     case ACPI_CPU_FLAGS_OFFSET_RW: /* pack and return is_* fields */
-        val |= cdev->cpu ? 1 : 0;
+        val |= check_cpu_enabled_status(DEVICE(cdev->cpu)) ? 1 : 0;
         val |= cdev->is_inserting ? 2 : 0;
         val |= cdev->is_removing  ? 4 : 0;
         val |= cdev->fw_remove  ? 16 : 0;
+        val |= cdev->cpu ? 32 : 0;
         trace_cpuhp_acpi_read_flags(cpu_st->selector, val);
         break;
     case ACPI_CPU_CMD_DATA_OFFSET_RW:
@@ -349,6 +362,7 @@ const VMStateDescription vmstate_cpu_hotplug = {
 #define CPU_REMOVE_EVENT  "CRMV"
 #define CPU_EJECT_EVENT   "CEJ0"
 #define CPU_FW_EJECT_EVENT "CEJF"
+#define CPU_PRESENT       "CPRS"
 
 void build_cpus_aml(Aml *table, MachineState *machine, CPUHotplugFeatures opts,
                     build_madt_cpu_fn build_madt_cpu, hwaddr base_addr,
@@ -409,7 +423,9 @@ void build_cpus_aml(Aml *table, MachineState *machine, CPUHotplugFeatures opts,
         aml_append(field, aml_named_field(CPU_EJECT_EVENT, 1));
         /* tell firmware to do device eject, write only */
         aml_append(field, aml_named_field(CPU_FW_EJECT_EVENT, 1));
-        aml_append(field, aml_reserved_field(3));
+        /* 1 if present, read only */
+        aml_append(field, aml_named_field(CPU_PRESENT, 1));
+        aml_append(field, aml_reserved_field(2));
         aml_append(field, aml_named_field(CPU_COMMAND, 8));
         aml_append(cpu_ctrl_dev, field);
 
@@ -439,6 +455,7 @@ void build_cpus_aml(Aml *table, MachineState *machine, CPUHotplugFeatures opts,
         Aml *ctrl_lock = aml_name("%s.%s", cphp_res_path, CPU_LOCK);
         Aml *cpu_selector = aml_name("%s.%s", cphp_res_path, CPU_SELECTOR);
         Aml *is_enabled = aml_name("%s.%s", cphp_res_path, CPU_ENABLED);
+        Aml *is_present = aml_name("%s.%s", cphp_res_path, CPU_PRESENT);
         Aml *cpu_cmd = aml_name("%s.%s", cphp_res_path, CPU_COMMAND);
         Aml *cpu_data = aml_name("%s.%s", cphp_res_path, CPU_DATA);
         Aml *ins_evt = aml_name("%s.%s", cphp_res_path, CPU_INSERT_EVENT);
@@ -467,13 +484,26 @@ void build_cpus_aml(Aml *table, MachineState *machine, CPUHotplugFeatures opts,
         {
             Aml *idx = aml_arg(0);
             Aml *sta = aml_local(0);
+            Aml *ifctx2;
+            Aml *else_ctx;
 
             aml_append(method, aml_acquire(ctrl_lock, 0xFFFF));
             aml_append(method, aml_store(idx, cpu_selector));
             aml_append(method, aml_store(zero, sta));
-            ifctx = aml_if(aml_equal(is_enabled, one));
+            ifctx = aml_if(aml_equal(is_present, one));
             {
-                aml_append(ifctx, aml_store(aml_int(0xF), sta));
+                ifctx2 = aml_if(aml_equal(is_enabled, one));
+                {
+                    /* cpu is present and enabled */
+                    aml_append(ifctx2, aml_store(aml_int(0xF), sta));
+                }
+                aml_append(ifctx, ifctx2);
+                else_ctx = aml_else();
+                {
+                    /* cpu is present but disabled */
+                    aml_append(else_ctx, aml_store(aml_int(0xD), sta));
+                }
+                aml_append(ifctx, else_ctx);
             }
             aml_append(method, ifctx);
             aml_append(method, aml_release(ctrl_lock));
diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
index e7de77dc6d..db8a6fbc6e 100644
--- a/include/hw/core/cpu.h
+++ b/include/hw/core/cpu.h
@@ -159,6 +159,7 @@ struct CPUClass {
     void (*query_cpu_fast)(CPUState *cpu, CpuInfoFast *value);
     int64_t (*get_arch_id)(CPUState *cpu);
     bool (*cpu_persistent_status)(CPUState *cpu);
+    bool (*cpu_enabled_status)(CPUState *cpu);
     void (*set_pc)(CPUState *cpu, vaddr value);
     vaddr (*get_pc)(CPUState *cpu);
     int (*gdb_read_register)(CPUState *cpu, GByteArray *buf, int reg);
-- 
2.34.1