From 3780dddd4fc8f0471525c50893e24846d1474692 Mon Sep 17 00:00:00 2001
From: Salil Mehta <[email protected]>
Date: Tue, 8 Aug 2023 00:43:18 +0000
Subject: [PATCH] arm/virt: Make ARM vCPU *present* status ACPI *persistent*

ARM arch does not allow CPUs presence to be changed [1] after kernel has booted.
Hence, firmware/ACPI/Qemu must ensure persistent view of the vCPUs to the Guest
kernel even when they are not present in the QoM i.e. are unplugged or are
yet-to-be-plugged

References:
[1] Check comment 5 in the bugzilla entry
   Link: https://bugzilla.tianocore.org/show_bug.cgi?id=4481#c5

Signed-off-by: Salil Mehta <[email protected]>
---
 cpu-common.c          |  6 ++++++
 hw/arm/virt.c         |  7 +++++++
 include/hw/core/cpu.h | 20 ++++++++++++++++++++
 3 files changed, 33 insertions(+)

diff --git a/cpu-common.c b/cpu-common.c
index d041a351ab..da52e45760 100644
--- a/cpu-common.c
+++ b/cpu-common.c
@@ -128,6 +128,12 @@ bool qemu_enabled_cpu(CPUState *cpu)
     return cpu && !cpu->disabled;
 }
 
+bool qemu_persistent_cpu(CPUState *cpu)
+{
+    /* cpu state can be faked to the guest via acpi */
+    return cpu->acpi_persistent;
+}
+
 uint64_t qemu_get_cpu_archid(int cpu_index)
 {
     MachineState *ms = MACHINE(qdev_get_machine());
diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index 818398e753..91b2653c03 100644
--- a/hw/arm/virt.c
+++ b/hw/arm/virt.c
@@ -3104,6 +3104,13 @@ static void virt_cpu_pre_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
         return;
     }
     virt_cpu_set_properties(OBJECT(cs), cpu_slot, errp);
+
+    /*
+     * To give persistent presence view of vCPUs to the guest, ACPI might need
+     * to fake the presence of the vCPUs to the guest but keep them disabled.
+     * This shall be used during the init of ACPI Hotplug state and hot-unplug
+     */
+     cs->acpi_persistent = true;
 }
 
 static void virt_cpu_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
index fdfb952259..0ca778eb75 100644
--- a/include/hw/core/cpu.h
+++ b/include/hw/core/cpu.h
@@ -550,6 +550,13 @@ struct CPUState {
      * By default every CPUState is enabled as of now across all archs.
      */
     bool disabled;
+    /*
+     * On certain architectures, to give persistent view of the 'presence' of
+     * vCPUs to the guest, ACPI might need to fake the 'presence' of the vCPUs
+     * but keep them ACPI disabled to the guest. This is done by returning
+     * _STA.PRES=True and _STA.Ena=False for the unplugged vCPUs in QEMU QoM.
+     */
+    bool acpi_persistent;
     /* TODO Move common fields from CPUArchState here. */
     int cpu_index;
     int cluster_index;
@@ -957,6 +964,19 @@ bool qemu_present_cpu(CPUState *cpu);
  */
 bool qemu_enabled_cpu(CPUState *cpu);
 
+/**
+ * qemu_persistent_cpu:
+ * @cpu: The vCPU to check
+ *
+ * Checks if the vCPU state should always be reflected as *present* via ACPI
+ * to the Guest. By default, this is False on all architectures and has to be
+ * explicity set during initialization.
+ *
+ * Returns: True if it is ACPI 'persistent' CPU
+ *
+ */
+bool qemu_persistent_cpu(CPUState *cpu);
+
 /**
  * qemu_get_cpu_archid:
  * @cpu_index: possible vCPU for which arch-id needs to be retreived
-- 
2.27.0