[PATCH v2 06/16] OvmfPkg/CpuHotplugSmm: introduce skeleton for CPU Hotplug SMM driver

["Laszlo Ersek" <lersek@redhat.com>]

Add a new SMM driver skeleton that registers a root SMI handler, and
checks if the SMI control value (written to 0xB2) indicates a CPU hotplug
SMI.

QEMU's ACPI payload will cause the OS to raise a broadcast SMI when a CPU
hotplug event occurs, namely by writing value 4 to IO Port 0xB2. In other
words, control value 4 is now allocated for this purpose; introduce the
ICH9_APM_CNT_CPU_HOTPLUG macro for it.

The standard identifiers in this driver use the new MM (Management Mode)
terminology from the PI spec, not the earlier SMM (System Management Mode)
terms.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1512
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---

Notes:
    v2:
    
    - Pick up Ard's Acked-by, which is conditional on approval from Intel
      reviewers on Cc. (I'd like to save Ard the churn of re-acking
      unmodified patches.)

 OvmfPkg/OvmfPkgIa32.dsc                       |   1 +
 OvmfPkg/OvmfPkgIa32X64.dsc                    |   1 +
 OvmfPkg/OvmfPkgX64.dsc                        |   1 +
 OvmfPkg/OvmfPkgIa32.fdf                       |   1 +
 OvmfPkg/OvmfPkgIa32X64.fdf                    |   1 +
 OvmfPkg/OvmfPkgX64.fdf                        |   1 +
 OvmfPkg/Include/IndustryStandard/Q35MchIch9.h |   5 +-
 OvmfPkg/CpuHotplugSmm/CpuHotplugSmm.inf       |  48 +++++
 OvmfPkg/CpuHotplugSmm/CpuHotplug.c            | 191 ++++++++++++++++++++
 9 files changed, 248 insertions(+), 2 deletions(-)

diff --git a/OvmfPkg/OvmfPkgIa32.dsc b/OvmfPkg/OvmfPkgIa32.dsc
index 8c065ca7cec9..78310da44a5f 100644
--- a/OvmfPkg/OvmfPkgIa32.dsc
+++ b/OvmfPkg/OvmfPkgIa32.dsc
@@ -851,44 +851,45 @@ [Components]
 
   OvmfPkg/PlatformDxe/Platform.inf
   OvmfPkg/IoMmuDxe/IoMmuDxe.inf
 
 !if $(SMM_REQUIRE) == TRUE
   OvmfPkg/SmmAccess/SmmAccess2Dxe.inf
   OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.inf
   UefiCpuPkg/CpuS3DataDxe/CpuS3DataDxe.inf
 
   #
   # SMM Initial Program Load (a DXE_RUNTIME_DRIVER)
   #
   MdeModulePkg/Core/PiSmmCore/PiSmmIpl.inf
 
   #
   # SMM_CORE
   #
   MdeModulePkg/Core/PiSmmCore/PiSmmCore.inf
 
   #
   # Privileged drivers (DXE_SMM_DRIVER modules)
   #
+  OvmfPkg/CpuHotplugSmm/CpuHotplugSmm.inf
   UefiCpuPkg/CpuIo2Smm/CpuIo2Smm.inf
   MdeModulePkg/Universal/LockBox/SmmLockBox/SmmLockBox.inf {
     <LibraryClasses>
       LockBoxLib|MdeModulePkg/Library/SmmLockBoxLib/SmmLockBoxSmmLib.inf
   }
   UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.inf {
     <LibraryClasses>
       SmmCpuPlatformHookLib|OvmfPkg/Library/SmmCpuPlatformHookLibQemu/SmmCpuPlatformHookLibQemu.inf
       SmmCpuFeaturesLib|OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.inf
   }
 
   #
   # Variable driver stack (SMM)
   #
   OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesSmm.inf
   MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm.inf
   MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmm.inf {
     <LibraryClasses>
       NULL|MdeModulePkg/Library/VarCheckUefiLib/VarCheckUefiLib.inf
   }
   MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmmRuntimeDxe.inf
 
diff --git a/OvmfPkg/OvmfPkgIa32X64.dsc b/OvmfPkg/OvmfPkgIa32X64.dsc
index 944b785e61a9..428578a4f839 100644
--- a/OvmfPkg/OvmfPkgIa32X64.dsc
+++ b/OvmfPkg/OvmfPkgIa32X64.dsc
@@ -865,44 +865,45 @@ [Components.X64]
   OvmfPkg/PlatformDxe/Platform.inf
   OvmfPkg/AmdSevDxe/AmdSevDxe.inf
   OvmfPkg/IoMmuDxe/IoMmuDxe.inf
 
 !if $(SMM_REQUIRE) == TRUE
   OvmfPkg/SmmAccess/SmmAccess2Dxe.inf
   OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.inf
   UefiCpuPkg/CpuS3DataDxe/CpuS3DataDxe.inf
 
   #
   # SMM Initial Program Load (a DXE_RUNTIME_DRIVER)
   #
   MdeModulePkg/Core/PiSmmCore/PiSmmIpl.inf
 
   #
   # SMM_CORE
   #
   MdeModulePkg/Core/PiSmmCore/PiSmmCore.inf
 
   #
   # Privileged drivers (DXE_SMM_DRIVER modules)
   #
+  OvmfPkg/CpuHotplugSmm/CpuHotplugSmm.inf
   UefiCpuPkg/CpuIo2Smm/CpuIo2Smm.inf
   MdeModulePkg/Universal/LockBox/SmmLockBox/SmmLockBox.inf {
     <LibraryClasses>
       LockBoxLib|MdeModulePkg/Library/SmmLockBoxLib/SmmLockBoxSmmLib.inf
   }
   UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.inf {
     <LibraryClasses>
       SmmCpuPlatformHookLib|OvmfPkg/Library/SmmCpuPlatformHookLibQemu/SmmCpuPlatformHookLibQemu.inf
       SmmCpuFeaturesLib|OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.inf
   }
 
   #
   # Variable driver stack (SMM)
   #
   OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesSmm.inf
   MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm.inf
   MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmm.inf {
     <LibraryClasses>
       NULL|MdeModulePkg/Library/VarCheckUefiLib/VarCheckUefiLib.inf
   }
   MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmmRuntimeDxe.inf
 
diff --git a/OvmfPkg/OvmfPkgX64.dsc b/OvmfPkg/OvmfPkgX64.dsc
index 8de0f7179784..73b92f259201 100644
--- a/OvmfPkg/OvmfPkgX64.dsc
+++ b/OvmfPkg/OvmfPkgX64.dsc
@@ -863,44 +863,45 @@ [Components]
   OvmfPkg/PlatformDxe/Platform.inf
   OvmfPkg/AmdSevDxe/AmdSevDxe.inf
   OvmfPkg/IoMmuDxe/IoMmuDxe.inf
 
 !if $(SMM_REQUIRE) == TRUE
   OvmfPkg/SmmAccess/SmmAccess2Dxe.inf
   OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.inf
   UefiCpuPkg/CpuS3DataDxe/CpuS3DataDxe.inf
 
   #
   # SMM Initial Program Load (a DXE_RUNTIME_DRIVER)
   #
   MdeModulePkg/Core/PiSmmCore/PiSmmIpl.inf
 
   #
   # SMM_CORE
   #
   MdeModulePkg/Core/PiSmmCore/PiSmmCore.inf
 
   #
   # Privileged drivers (DXE_SMM_DRIVER modules)
   #
+  OvmfPkg/CpuHotplugSmm/CpuHotplugSmm.inf
   UefiCpuPkg/CpuIo2Smm/CpuIo2Smm.inf
   MdeModulePkg/Universal/LockBox/SmmLockBox/SmmLockBox.inf {
     <LibraryClasses>
       LockBoxLib|MdeModulePkg/Library/SmmLockBoxLib/SmmLockBoxSmmLib.inf
   }
   UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.inf {
     <LibraryClasses>
       SmmCpuPlatformHookLib|OvmfPkg/Library/SmmCpuPlatformHookLibQemu/SmmCpuPlatformHookLibQemu.inf
       SmmCpuFeaturesLib|OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.inf
   }
 
   #
   # Variable driver stack (SMM)
   #
   OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesSmm.inf
   MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm.inf
   MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmm.inf {
     <LibraryClasses>
       NULL|MdeModulePkg/Library/VarCheckUefiLib/VarCheckUefiLib.inf
   }
   MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmmRuntimeDxe.inf
 
diff --git a/OvmfPkg/OvmfPkgIa32.fdf b/OvmfPkg/OvmfPkgIa32.fdf
index 63607551ed75..61b891765c56 100644
--- a/OvmfPkg/OvmfPkgIa32.fdf
+++ b/OvmfPkg/OvmfPkgIa32.fdf
@@ -301,44 +301,45 @@ [FV.DXEFV]
 INF  MdeModulePkg/Bus/Usb/UsbKbDxe/UsbKbDxe.inf
 INF  MdeModulePkg/Bus/Usb/UsbMassStorageDxe/UsbMassStorageDxe.inf
 
 !ifdef $(CSM_ENABLE)
 INF  OvmfPkg/Csm/BiosThunk/VideoDxe/VideoDxe.inf
 INF  OvmfPkg/Csm/LegacyBiosDxe/LegacyBiosDxe.inf
 INF  RuleOverride=CSM OvmfPkg/Csm/Csm16/Csm16.inf
 !else
 INF  OvmfPkg/QemuVideoDxe/QemuVideoDxe.inf
 !endif
 
 INF  OvmfPkg/QemuRamfbDxe/QemuRamfbDxe.inf
 INF  OvmfPkg/VirtioGpuDxe/VirtioGpu.inf
 INF  OvmfPkg/PlatformDxe/Platform.inf
 INF  OvmfPkg/IoMmuDxe/IoMmuDxe.inf
 
 !if $(SMM_REQUIRE) == TRUE
 INF  OvmfPkg/SmmAccess/SmmAccess2Dxe.inf
 INF  OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.inf
 INF  UefiCpuPkg/CpuS3DataDxe/CpuS3DataDxe.inf
 INF  MdeModulePkg/Core/PiSmmCore/PiSmmIpl.inf
 INF  MdeModulePkg/Core/PiSmmCore/PiSmmCore.inf
+INF  OvmfPkg/CpuHotplugSmm/CpuHotplugSmm.inf
 INF  UefiCpuPkg/CpuIo2Smm/CpuIo2Smm.inf
 INF  MdeModulePkg/Universal/LockBox/SmmLockBox/SmmLockBox.inf
 INF  UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.inf
 
 #
 # Variable driver stack (SMM)
 #
 INF  OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesSmm.inf
 INF  MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm.inf
 INF  MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmm.inf
 INF  MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmmRuntimeDxe.inf
 
 !else
 
 #
 # Variable driver stack (non-SMM)
 #
 INF  OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesRuntimeDxe.inf
 INF  OvmfPkg/EmuVariableFvbRuntimeDxe/Fvb.inf
 INF  MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteDxe.inf
 INF  MdeModulePkg/Universal/Variable/RuntimeDxe/VariableRuntimeDxe.inf
 !endif
diff --git a/OvmfPkg/OvmfPkgIa32X64.fdf b/OvmfPkg/OvmfPkgIa32X64.fdf
index 0488e5d95ffe..501b4fcb7b67 100644
--- a/OvmfPkg/OvmfPkgIa32X64.fdf
+++ b/OvmfPkg/OvmfPkgIa32X64.fdf
@@ -308,44 +308,45 @@ [FV.DXEFV]
 INF  MdeModulePkg/Bus/Usb/UsbMassStorageDxe/UsbMassStorageDxe.inf
 
 !ifdef $(CSM_ENABLE)
 INF  OvmfPkg/Csm/BiosThunk/VideoDxe/VideoDxe.inf
 INF  OvmfPkg/Csm/LegacyBiosDxe/LegacyBiosDxe.inf
 INF  RuleOverride=CSM OvmfPkg/Csm/Csm16/Csm16.inf
 !else
 INF  OvmfPkg/QemuVideoDxe/QemuVideoDxe.inf
 !endif
 
 INF  OvmfPkg/QemuRamfbDxe/QemuRamfbDxe.inf
 INF  OvmfPkg/VirtioGpuDxe/VirtioGpu.inf
 INF  OvmfPkg/PlatformDxe/Platform.inf
 INF  OvmfPkg/AmdSevDxe/AmdSevDxe.inf
 INF  OvmfPkg/IoMmuDxe/IoMmuDxe.inf
 
 !if $(SMM_REQUIRE) == TRUE
 INF  OvmfPkg/SmmAccess/SmmAccess2Dxe.inf
 INF  OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.inf
 INF  UefiCpuPkg/CpuS3DataDxe/CpuS3DataDxe.inf
 INF  MdeModulePkg/Core/PiSmmCore/PiSmmIpl.inf
 INF  MdeModulePkg/Core/PiSmmCore/PiSmmCore.inf
+INF  OvmfPkg/CpuHotplugSmm/CpuHotplugSmm.inf
 INF  UefiCpuPkg/CpuIo2Smm/CpuIo2Smm.inf
 INF  MdeModulePkg/Universal/LockBox/SmmLockBox/SmmLockBox.inf
 INF  UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.inf
 
 #
 # Variable driver stack (SMM)
 #
 INF  OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesSmm.inf
 INF  MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm.inf
 INF  MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmm.inf
 INF  MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmmRuntimeDxe.inf
 
 !else
 
 #
 # Variable driver stack (non-SMM)
 #
 INF  OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesRuntimeDxe.inf
 INF  OvmfPkg/EmuVariableFvbRuntimeDxe/Fvb.inf
 INF  MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteDxe.inf
 INF  MdeModulePkg/Universal/Variable/RuntimeDxe/VariableRuntimeDxe.inf
 !endif
diff --git a/OvmfPkg/OvmfPkgX64.fdf b/OvmfPkg/OvmfPkgX64.fdf
index 0488e5d95ffe..501b4fcb7b67 100644
--- a/OvmfPkg/OvmfPkgX64.fdf
+++ b/OvmfPkg/OvmfPkgX64.fdf
@@ -308,44 +308,45 @@ [FV.DXEFV]
 INF  MdeModulePkg/Bus/Usb/UsbMassStorageDxe/UsbMassStorageDxe.inf
 
 !ifdef $(CSM_ENABLE)
 INF  OvmfPkg/Csm/BiosThunk/VideoDxe/VideoDxe.inf
 INF  OvmfPkg/Csm/LegacyBiosDxe/LegacyBiosDxe.inf
 INF  RuleOverride=CSM OvmfPkg/Csm/Csm16/Csm16.inf
 !else
 INF  OvmfPkg/QemuVideoDxe/QemuVideoDxe.inf
 !endif
 
 INF  OvmfPkg/QemuRamfbDxe/QemuRamfbDxe.inf
 INF  OvmfPkg/VirtioGpuDxe/VirtioGpu.inf
 INF  OvmfPkg/PlatformDxe/Platform.inf
 INF  OvmfPkg/AmdSevDxe/AmdSevDxe.inf
 INF  OvmfPkg/IoMmuDxe/IoMmuDxe.inf
 
 !if $(SMM_REQUIRE) == TRUE
 INF  OvmfPkg/SmmAccess/SmmAccess2Dxe.inf
 INF  OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.inf
 INF  UefiCpuPkg/CpuS3DataDxe/CpuS3DataDxe.inf
 INF  MdeModulePkg/Core/PiSmmCore/PiSmmIpl.inf
 INF  MdeModulePkg/Core/PiSmmCore/PiSmmCore.inf
+INF  OvmfPkg/CpuHotplugSmm/CpuHotplugSmm.inf
 INF  UefiCpuPkg/CpuIo2Smm/CpuIo2Smm.inf
 INF  MdeModulePkg/Universal/LockBox/SmmLockBox/SmmLockBox.inf
 INF  UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.inf
 
 #
 # Variable driver stack (SMM)
 #
 INF  OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesSmm.inf
 INF  MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm.inf
 INF  MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmm.inf
 INF  MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmmRuntimeDxe.inf
 
 !else
 
 #
 # Variable driver stack (non-SMM)
 #
 INF  OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesRuntimeDxe.inf
 INF  OvmfPkg/EmuVariableFvbRuntimeDxe/Fvb.inf
 INF  MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteDxe.inf
 INF  MdeModulePkg/Universal/Variable/RuntimeDxe/VariableRuntimeDxe.inf
 !endif
diff --git a/OvmfPkg/Include/IndustryStandard/Q35MchIch9.h b/OvmfPkg/Include/IndustryStandard/Q35MchIch9.h
index cb705fee92ca..73db4b59a111 100644
--- a/OvmfPkg/Include/IndustryStandard/Q35MchIch9.h
+++ b/OvmfPkg/Include/IndustryStandard/Q35MchIch9.h
@@ -90,37 +90,38 @@
 #define POWER_MGMT_REGISTER_Q35(Offset) \
   PCI_LIB_ADDRESS (0, 0x1f, 0, (Offset))
 
 #define POWER_MGMT_REGISTER_Q35_EFI_PCI_ADDRESS(Offset) \
   EFI_PCI_ADDRESS (0, 0x1f, 0, (Offset))
 
 #define ICH9_PMBASE               0x40
 #define ICH9_PMBASE_MASK            (BIT15 | BIT14 | BIT13 | BIT12 | BIT11 | \
                                      BIT10 | BIT9  | BIT8  | BIT7)
 
 #define ICH9_ACPI_CNTL            0x44
 #define ICH9_ACPI_CNTL_ACPI_EN      BIT7
 
 #define ICH9_GEN_PMCON_1          0xA0
 #define ICH9_GEN_PMCON_1_SMI_LOCK   BIT4
 
 #define ICH9_RCBA                 0xF0
 #define ICH9_RCBA_EN                BIT0
 
 //
 // IO ports
 //
-#define ICH9_APM_CNT 0xB2
-#define ICH9_APM_STS 0xB3
+#define ICH9_APM_CNT              0xB2
+#define ICH9_APM_CNT_CPU_HOTPLUG    0x04
+#define ICH9_APM_STS              0xB3
 
 #define ICH9_CPU_HOTPLUG_BASE 0x0CD8
 
 //
 // IO ports relative to PMBASE
 //
 #define ICH9_PMBASE_OFS_SMI_EN 0x30
 #define ICH9_SMI_EN_APMC_EN      BIT5
 #define ICH9_SMI_EN_GBL_SMI_EN   BIT0
 
 #define ICH9_ROOT_COMPLEX_BASE 0xFED1C000
 
 #endif
diff --git a/OvmfPkg/CpuHotplugSmm/CpuHotplugSmm.inf b/OvmfPkg/CpuHotplugSmm/CpuHotplugSmm.inf
new file mode 100644
index 000000000000..fa70858a8dab
--- /dev/null
+++ b/OvmfPkg/CpuHotplugSmm/CpuHotplugSmm.inf
@@ -0,0 +1,48 @@
+## @file
+# Root SMI handler for VCPU hotplug SMIs.
+#
+# Copyright (c) 2020, Red Hat, Inc.
+#
+# SPDX-License-Identifier: BSD-2-Clause-Patent
+##
+
+[Defines]
+  INF_VERSION                = 1.29
+  PI_SPECIFICATION_VERSION   = 0x00010046                            # PI-1.7.0
+  BASE_NAME                  = CpuHotplugSmm
+  FILE_GUID                  = 84EEA114-C6BE-4445-8F90-51D97863E363
+  MODULE_TYPE                = DXE_SMM_DRIVER
+  ENTRY_POINT                = CpuHotplugEntry
+
+#
+# The following information is for reference only and not required by the build
+# tools.
+#
+# VALID_ARCHITECTURES        = IA32 X64
+#
+
+[Sources]
+  CpuHotplug.c
+
+[Packages]
+  MdePkg/MdePkg.dec
+  OvmfPkg/OvmfPkg.dec
+
+[LibraryClasses]
+  BaseLib
+  DebugLib
+  MmServicesTableLib
+  PcdLib
+  UefiDriverEntryPoint
+
+[Protocols]
+  gEfiMmCpuIoProtocolGuid                                           ## CONSUMES
+
+[Pcd]
+  gUefiOvmfPkgTokenSpaceGuid.PcdQ35SmramAtDefaultSmbase             ## CONSUMES
+
+[FeaturePcd]
+  gUefiOvmfPkgTokenSpaceGuid.PcdSmmSmramRequire                     ## CONSUMES
+
+[Depex]
+  gEfiMmCpuIoProtocolGuid
diff --git a/OvmfPkg/CpuHotplugSmm/CpuHotplug.c b/OvmfPkg/CpuHotplugSmm/CpuHotplug.c
new file mode 100644
index 000000000000..fd09403eabf3
--- /dev/null
+++ b/OvmfPkg/CpuHotplugSmm/CpuHotplug.c
@@ -0,0 +1,191 @@
+/** @file
+  Root SMI handler for VCPU hotplug SMIs.
+
+  Copyright (c) 2020, Red Hat, Inc.
+
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+**/
+
+#include <IndustryStandard/Q35MchIch9.h>     // ICH9_APM_CNT
+#include <Library/BaseLib.h>                 // CpuDeadLoop()
+#include <Library/DebugLib.h>                // ASSERT()
+#include <Library/MmServicesTableLib.h>      // gMmst
+#include <Library/PcdLib.h>                  // PcdGetBool()
+#include <Protocol/MmCpuIo.h>                // EFI_MM_CPU_IO_PROTOCOL
+#include <Uefi/UefiBaseType.h>               // EFI_STATUS
+
+//
+// We use this protocol for accessing IO Ports.
+//
+STATIC EFI_MM_CPU_IO_PROTOCOL *mMmCpuIo;
+//
+// Represents the registration of the CPU Hotplug MMI handler.
+//
+STATIC EFI_HANDLE mDispatchHandle;
+
+
+/**
+  CPU Hotplug MMI handler function.
+
+  This is a root MMI handler.
+
+  @param[in] DispatchHandle      The unique handle assigned to this handler by
+                                 EFI_MM_SYSTEM_TABLE.MmiHandlerRegister().
+
+  @param[in] Context             Context passed in by
+                                 EFI_MM_SYSTEM_TABLE.MmiManage(). Due to
+                                 CpuHotplugMmi() being a root MMI handler,
+                                 Context is ASSERT()ed to be NULL.
+
+  @param[in,out] CommBuffer      Ignored, due to CpuHotplugMmi() being a root
+                                 MMI handler.
+
+  @param[in,out] CommBufferSize  Ignored, due to CpuHotplugMmi() being a root
+                                 MMI handler.
+
+  @retval EFI_SUCCESS                       The MMI was handled and the MMI
+                                            source was quiesced. When returned
+                                            by a non-root MMI handler,
+                                            EFI_SUCCESS terminates the
+                                            processing of MMI handlers in
+                                            EFI_MM_SYSTEM_TABLE.MmiManage().
+                                            For a root MMI handler (i.e., for
+                                            the present function too),
+                                            EFI_SUCCESS behaves identically to
+                                            EFI_WARN_INTERRUPT_SOURCE_QUIESCED,
+                                            as further root MMI handlers are
+                                            going to be called by
+                                            EFI_MM_SYSTEM_TABLE.MmiManage()
+                                            anyway.
+
+  @retval EFI_WARN_INTERRUPT_SOURCE_QUIESCED  The MMI source has been quiesced,
+                                              but other handlers should still
+                                              be called.
+
+  @retval EFI_WARN_INTERRUPT_SOURCE_PENDING   The MMI source is still pending,
+                                              and other handlers should still
+                                              be called.
+
+  @retval EFI_INTERRUPT_PENDING               The MMI source could not be
+                                              quiesced.
+**/
+STATIC
+EFI_STATUS
+EFIAPI
+CpuHotplugMmi (
+  IN EFI_HANDLE DispatchHandle,
+  IN CONST VOID *Context        OPTIONAL,
+  IN OUT VOID   *CommBuffer     OPTIONAL,
+  IN OUT UINTN  *CommBufferSize OPTIONAL
+  )
+{
+  EFI_STATUS Status;
+  UINT8      ApmControl;
+
+  //
+  // Assert that we are entering this function due to our root MMI handler
+  // registration.
+  //
+  ASSERT (DispatchHandle == mDispatchHandle);
+  //
+  // When MmiManage() is invoked to process root MMI handlers, the caller (the
+  // MM Core) is expected to pass in a NULL Context. MmiManage() then passes
+  // the same NULL Context to individual handlers.
+  //
+  ASSERT (Context == NULL);
+  //
+  // Read the MMI command value from the APM Control Port, to see if this is an
+  // MMI we should care about.
+  //
+  Status = mMmCpuIo->Io.Read (mMmCpuIo, MM_IO_UINT8, ICH9_APM_CNT, 1,
+                          &ApmControl);
+  if (EFI_ERROR (Status)) {
+    DEBUG ((DEBUG_ERROR, "%a: failed to read ICH9_APM_CNT: %r\n", __FUNCTION__,
+      Status));
+    //
+    // We couldn't even determine if the MMI was for us or not.
+    //
+    goto Fatal;
+  }
+
+  if (ApmControl != ICH9_APM_CNT_CPU_HOTPLUG) {
+    //
+    // The MMI is not for us.
+    //
+    return EFI_WARN_INTERRUPT_SOURCE_QUIESCED;
+  }
+
+  //
+  // We've handled this MMI.
+  //
+  return EFI_SUCCESS;
+
+Fatal:
+  ASSERT (FALSE);
+  CpuDeadLoop ();
+  //
+  // We couldn't handle this MMI.
+  //
+  return EFI_INTERRUPT_PENDING;
+}
+
+
+//
+// Entry point function of this driver.
+//
+EFI_STATUS
+EFIAPI
+CpuHotplugEntry (
+  IN EFI_HANDLE       ImageHandle,
+  IN EFI_SYSTEM_TABLE *SystemTable
+  )
+{
+  EFI_STATUS Status;
+
+  //
+  // This module should only be included when SMM support is required.
+  //
+  ASSERT (FeaturePcdGet (PcdSmmSmramRequire));
+  //
+  // This driver depends on the dynamically detected "SMRAM at default SMBASE"
+  // feature.
+  //
+  if (!PcdGetBool (PcdQ35SmramAtDefaultSmbase)) {
+    return EFI_UNSUPPORTED;
+  }
+
+  //
+  // Errors from here on are fatal; we cannot allow the boot to proceed if we
+  // can't set up this driver to handle CPU hotplug.
+  //
+  // First, collect the protocols needed later. All of these protocols are
+  // listed in our module DEPEX.
+  //
+  Status = gMmst->MmLocateProtocol (&gEfiMmCpuIoProtocolGuid,
+                    NULL /* Registration */, (VOID **)&mMmCpuIo);
+  if (EFI_ERROR (Status)) {
+    DEBUG ((DEBUG_ERROR, "%a: locate MmCpuIo: %r\n", __FUNCTION__, Status));
+    goto Fatal;
+  }
+
+  //
+  // Register the handler for the CPU Hotplug MMI.
+  //
+  Status = gMmst->MmiHandlerRegister (
+                    CpuHotplugMmi,
+                    NULL,            // HandlerType: root MMI handler
+                    &mDispatchHandle
+                    );
+  if (EFI_ERROR (Status)) {
+    DEBUG ((DEBUG_ERROR, "%a: MmiHandlerRegister(): %r\n", __FUNCTION__,
+      Status));
+    goto Fatal;
+  }
+
+  return EFI_SUCCESS;
+
+Fatal:
+  ASSERT (FALSE);
+  CpuDeadLoop ();
+  return Status;
+}
-- 
2.19.1.3.g30247aa5d201