[PATCH RFC V2 37/37] hw/arm/virt: Expose cold-booted CPUs as MADT GICC Enabled

[Salil Mehta via <qemu-devel@nongnu.org>]

Hotpluggable CPUs MUST be exposed as 'online-capable' as per the new change. But
cold booted CPUs if made 'online-capable' during boot time might not get
detected in the legacy OS. Hence, can cause compatibility problems.

Original Change Link: https://bugzilla.tianocore.org/show_bug.cgi?id=3706

Specification change might take time and hence disabling the support of
unplugging any cold booted CPUs to preserve the compatibility with legacy OS.

Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
---
 hw/arm/virt-acpi-build.c | 19 ++++++++++++++-----
 hw/arm/virt.c            | 16 ++++++++++++++++
 include/hw/core/cpu.h    |  2 ++
 3 files changed, 32 insertions(+), 5 deletions(-)

diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
index 377450dd16..879c83a337 100644
--- a/hw/arm/virt-acpi-build.c
+++ b/hw/arm/virt-acpi-build.c
@@ -710,17 +710,26 @@ static uint32_t virt_acpi_get_gicc_flags(CPUState *cpu)
     }
 
     /*
-     * ARM GIC CPU Interface can be 'online-capable' or 'enabled' at boot
-     * We MUST set 'online-capable' Bit for all hotpluggable CPUs except the
-     * first/boot CPU. Cold-booted CPUs without 'Id' can also be unplugged.
-     * Though as-of-now this is only used as a debugging feature.
+     * ARM GIC CPU Interface can be 'online-capable' or 'enabled' at boot. We
+     * MUST set 'online-capable' bit for all hotpluggable CPUs.
+     * Change Link: https://bugzilla.tianocore.org/show_bug.cgi?id=3706
      *
      *   UEFI ACPI Specification 6.5
      *   Section: 5.2.12.14. GIC CPU Interface (GICC) Structure
      *   Table:   5.37 GICC CPU Interface Flags
      *   Link: https://uefi.org/specs/ACPI/6.5
+     *
+     * Cold-booted CPUs, except for the first/boot CPU, SHOULD be allowed to be
+     * hot(un)plug as well but for this to happen these MUST have
+     * 'online-capable' bit set. Later creates compatibility problem with legacy
+     * OS as it might ignore online-capable' bits during boot time and hence
+     * some CPUs might not get detected. To fix this MADT GIC CPU interface flag
+     * should be allowed to have both bits set i.e. 'online-capable' and
+     * 'Enabled' bits together. This change will require UEFI ACPI standard
+     * change. Till this happens exposing all cold-booted CPUs as 'enabled' only
+     *
      */
-    return cpu && !cpu->cpu_index ? 1 : (1 << 3);
+    return cpu && cpu->cold_booted ? 1 : (1 << 3);
 }
 
 static void
diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index e46f529801..3bfe9b9db3 100644
--- a/hw/arm/virt.c
+++ b/hw/arm/virt.c
@@ -3151,6 +3151,10 @@ static void virt_cpu_pre_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
      * This shall be used during the init of ACPI Hotplug state and hot-unplug
      */
      cs->acpi_persistent = true;
+
+    if (!dev->hotplugged) {
+        cs->cold_booted = true;
+    }
 }
 
 static void virt_cpu_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
@@ -3214,6 +3218,18 @@ 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 ffd815a0d8..f6b92a3285 100644
--- a/include/hw/core/cpu.h
+++ b/include/hw/core/cpu.h
@@ -441,6 +441,8 @@ struct CPUState {
     uint32_t can_do_io;
     int32_t exception_index;
 
+    bool cold_booted;
+
     AccelCPUState *accel;
     /* shared by kvm, hax and hvf */
     bool vcpu_dirty;
-- 
2.34.1