Hotpluggable vCPUs must be exposed as "online-capable" according to the new UEFI
specification [1][2]. However, marking cold-booted vCPUs as "online-capable"
during boot may cause them to go undetected by legacy operating systems,
potentially leading to compatibility issues. Hence, both 'online-capable' bit
and 'Enabled' bit in GIC CPU Interface flags should not be mutually exclusive as
they are now.

Since implementing this specification change may take time, it is necessary to
temporarily *disable* support for *unplugging* cold-booted vCPUs to maintain
compatibility with legacy OS environments.

As an alternative and temporary mitigation, we could introduce a property that
controls whether cold-booted vCPUs are marked as unpluggable. Community feedback
on this approach would be appreciated.

References:
[1] Original UEFI/ACPI proposed Change Bugzilla – TianoCore
    Link: https://bugzilla.tianocore.org/show_bug.cgi?id=3706
[2] Advanced Configuration and Power Interface (ACPI) Specification, Release 6.5, Aug 29, 2022
    Section: 5.2.12.14 GIC CPU Interface (GICC) Structure / Table 5.37: GICC CPU Interface Flags
    Link: https://uefi.org/sites/default/files/resources/ACPI_Spec_6_5_Aug29.pdf (Pages 138, 140)

Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
---
 hw/arm/virt.c         | 16 ++++++++++++++++
 include/hw/core/cpu.h |  2 ++
 2 files changed, 18 insertions(+)

diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index 15d9c2d8ac..d1172151be 100644
--- a/hw/arm/virt.c
+++ b/hw/arm/virt.c
@@ -3174,6 +3174,10 @@ static void virt_cpu_pre_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
         virt_update_gic(vms, cs, true);
         wire_gic_cpu_irqs(vms, cs);
     }
+
+    if (!dev->hotplugged) {
+        cs->cold_booted = true;
+    }
 }
 
 static void virt_cpu_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
@@ -3248,6 +3252,18 @@ static void virt_cpu_unplug_request(HotplugHandler *hotplug_dev,
         return;
     }
 
+    /*
+     * UEFI ACPI standard change is required to make both 'enabled' and the
+     * 'online-capable' bit co-exist instead of being mutually exclusive.
+     * check virt_acpi_get_gicc_flags() for more details.
+     *
+     * Disable the unplugging of cold-booted vCPUs as a temporary mitigation.
+     */
+    if (cs->cold_booted) {
+        error_setg(errp, "Hot-unplug of cold-booted CPU not supported!");
+        return;
+    }
+
     if (cs->cpu_index == first_cpu->cpu_index) {
         error_setg(errp, "Boot CPU(id%d=%d:%d:%d:%d) hot-unplug not supported",
                    first_cpu->cpu_index, cpu->socket_id, cpu->cluster_id,
diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
index 4a74c383ab..7b7ca2c32c 100644
--- a/include/hw/core/cpu.h
+++ b/include/hw/core/cpu.h
@@ -571,6 +571,8 @@ struct CPUState {
     uint32_t halted;
     int32_t exception_index;
 
+    bool cold_booted;
+
     AccelCPUState *accel;
 
     /* Used to keep track of an outstanding cpu throttle thread for migration
-- 
2.34.1