[PATCH V4] Introduce a new flag for i440fx to disable PCI hotplug on the root bus

[Ani Sinha]

We introduce a new global flag 'acpi-root-pci-hotplug' for i440fx with which
we can turn on or off PCI device hotplug on the root bus. This flag can be
used to prevent all PCI devices from getting hotplugged or unplugged from the
root PCI bus.
This feature is targetted mostly towards Windows VMs. It is useful in cases
where some hypervisor admins want to deploy guest VMs in a way so that the
users of the guest OSes are not able to hot-eject certain PCI devices from
the Windows system tray. Laine has explained the use case here in detail:
https://www.redhat.com/archives/libvir-list/2020-February/msg00110.html

Julia has resolved this issue for PCIE buses with the following commit:
530a0963184e57e71a5b538 ("pcie_root_port: Add hotplug disabling option")

This commit attempts to introduce similar behavior for PCI root buses used in
i440fx machine types (although in this case, we do not have a per-slot 
capability
to turn hotplug on or off).

Usage:
   -global PIIX4_PM.acpi-root-pci-hotplug=off

By default, this option is enabled which means that hotplug is turned on for the
PCI root bus.

The previously existing flag 'acpi-pci-hotplug-with-bridge-support' for PCI-PCI
bridges remain as is and can be used along with this new flag to control PCI
hotplug on PCI bridges.

This change has been tested using a Windows 2012R2 server guest image on a 
Ubuntu
18.04 host using the latest master qemu from upstream.

Signed-off-by: Ani Sinha <ani@anisinha.ca>
---
 hw/acpi/piix4.c      |  8 ++++++--
 hw/i386/acpi-build.c | 26 +++++++++++++++++++-------
 2 files changed, 25 insertions(+), 9 deletions(-)

Change Log:
V3..V4: Added use case detail in the commit log as per Igor's suggestion. Also  
added an example of qemu commandline showing how to use the option.

diff --git a/hw/acpi/piix4.c b/hw/acpi/piix4.c
index 26bac4f16c..4f436e5bf3 100644
--- a/hw/acpi/piix4.c
+++ b/hw/acpi/piix4.c
@@ -78,6 +78,7 @@ typedef struct PIIX4PMState {
 
     AcpiPciHpState acpi_pci_hotplug;
     bool use_acpi_hotplug_bridge;
+    bool use_acpi_root_pci_hotplug;
 
     uint8_t disable_s3;
     uint8_t disable_s4;
@@ -595,8 +596,9 @@ static void piix4_acpi_system_hot_add_init(MemoryRegion 
*parent,
                           "acpi-gpe0", GPE_LEN);
     memory_region_add_subregion(parent, GPE_BASE, &s->io_gpe);
 
-    acpi_pcihp_init(OBJECT(s), &s->acpi_pci_hotplug, bus, parent,
-                    s->use_acpi_hotplug_bridge);
+    if (s->use_acpi_hotplug_bridge || s->use_acpi_root_pci_hotplug)
+        acpi_pcihp_init(OBJECT(s), &s->acpi_pci_hotplug, bus, parent,
+                        s->use_acpi_hotplug_bridge);
 
     s->cpu_hotplug_legacy = true;
     object_property_add_bool(OBJECT(s), "cpu-hotplug-legacy",
@@ -635,6 +637,8 @@ static Property piix4_pm_properties[] = {
     DEFINE_PROP_UINT8(ACPI_PM_PROP_S4_VAL, PIIX4PMState, s4_val, 2),
     DEFINE_PROP_BOOL("acpi-pci-hotplug-with-bridge-support", PIIX4PMState,
                      use_acpi_hotplug_bridge, true),
+    DEFINE_PROP_BOOL("acpi-root-pci-hotplug", PIIX4PMState,
+                     use_acpi_root_pci_hotplug, true),
     DEFINE_PROP_BOOL("memory-hotplug-support", PIIX4PMState,
                      acpi_memory_hotplug.is_enabled, true),
     DEFINE_PROP_END_OF_LIST(),
diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
index b7bcbbbb2a..19a1702ad1 100644
--- a/hw/i386/acpi-build.c
+++ b/hw/i386/acpi-build.c
@@ -95,6 +95,7 @@ typedef struct AcpiPmInfo {
     bool s3_disabled;
     bool s4_disabled;
     bool pcihp_bridge_en;
+    bool pcihp_root_en;
     uint8_t s4_val;
     AcpiFadtData fadt;
     uint16_t cpu_hp_io_base;
@@ -245,6 +246,9 @@ static void acpi_get_pm_info(MachineState *machine, 
AcpiPmInfo *pm)
     pm->pcihp_bridge_en =
         object_property_get_bool(obj, "acpi-pci-hotplug-with-bridge-support",
                                  NULL);
+    pm->pcihp_root_en =
+        object_property_get_bool(obj, "acpi-root-pci-hotplug", NULL);
+
 }
 
 static void acpi_get_misc_info(AcpiMiscInfo *info)
@@ -337,15 +341,18 @@ static void build_append_pcihp_notify_entry(Aml *method, 
int slot)
 }
 
 static void build_append_pci_bus_devices(Aml *parent_scope, PCIBus *bus,
-                                         bool pcihp_bridge_en)
+                                         bool pcihp_bridge_en,
+                                         bool pcihp_root_en)
 {
     Aml *dev, *notify_method = NULL, *method;
     QObject *bsel;
     PCIBus *sec;
     int i;
+    bool root_bus = pci_bus_is_root(bus);
+    bool root_pcihp_disabled = (root_bus && !pcihp_root_en);
 
     bsel = object_property_get_qobject(OBJECT(bus), ACPI_PCIHP_PROP_BSEL, 
NULL);
-    if (bsel) {
+    if (bsel && !root_pcihp_disabled) {
         uint64_t bsel_val = qnum_get_uint(qobject_to(QNum, bsel));
 
         aml_append(parent_scope, aml_name_decl("BSEL", aml_int(bsel_val)));
@@ -361,6 +368,9 @@ static void build_append_pci_bus_devices(Aml *parent_scope, 
PCIBus *bus,
         bool bridge_in_acpi;
 
         if (!pdev) {
+            /* skip if pci hotplug for the root bus is disabled */
+            if (root_pcihp_disabled)
+                continue;
             if (bsel) { /* add hotplug slots for non present devices */
                 dev = aml_device("S%.02X", PCI_DEVFN(slot, 0));
                 aml_append(dev, aml_name_decl("_SUN", aml_int(slot)));
@@ -419,7 +429,7 @@ static void build_append_pci_bus_devices(Aml *parent_scope, 
PCIBus *bus,
             method = aml_method("_S3D", 0, AML_NOTSERIALIZED);
             aml_append(method, aml_return(aml_int(s3d)));
             aml_append(dev, method);
-        } else if (hotplug_enabled_dev) {
+        } else if (hotplug_enabled_dev && !root_pcihp_disabled) {
             /* add _SUN/_EJ0 to make slot hotpluggable  */
             aml_append(dev, aml_name_decl("_SUN", aml_int(slot)));
 
@@ -439,13 +449,14 @@ static void build_append_pci_bus_devices(Aml 
*parent_scope, PCIBus *bus,
              */
             PCIBus *sec_bus = pci_bridge_get_sec_bus(PCI_BRIDGE(pdev));
 
-            build_append_pci_bus_devices(dev, sec_bus, pcihp_bridge_en);
+            build_append_pci_bus_devices(dev, sec_bus, pcihp_bridge_en,
+                                         pcihp_root_en);
         }
         /* slot descriptor has been composed, add it into parent context */
         aml_append(parent_scope, dev);
     }
 
-    if (bsel) {
+    if (bsel && !root_pcihp_disabled) {
         aml_append(parent_scope, notify_method);
     }
 
@@ -455,7 +466,7 @@ static void build_append_pci_bus_devices(Aml *parent_scope, 
PCIBus *bus,
     method = aml_method("PCNT", 0, AML_NOTSERIALIZED);
 
     /* If bus supports hotplug select it and notify about local events */
-    if (bsel) {
+    if (bsel && !root_pcihp_disabled) {
         uint64_t bsel_val = qnum_get_uint(qobject_to(QNum, bsel));
 
         aml_append(method, aml_store(aml_int(bsel_val), aml_name("BNUM")));
@@ -1818,7 +1829,8 @@ build_dsdt(GArray *table_data, BIOSLinker *linker,
         if (bus) {
             Aml *scope = aml_scope("PCI0");
             /* Scan all PCI buses. Generate tables to support hotplug. */
-            build_append_pci_bus_devices(scope, bus, pm->pcihp_bridge_en);
+            build_append_pci_bus_devices(scope, bus, pm->pcihp_bridge_en,
+                                         pm->pcihp_root_en);
 
             if (TPM_IS_TIS_ISA(tpm)) {
                 if (misc->tpm_version == TPM_VERSION_2_0) {
-- 
2.17.1