[PATCH 3/3] ArmPlatformPkg: retreive error source descriptors from MM

["Omkar Anand Kulkarni" <omkar.kulkarni@arm.com>]

Add a driver that retreives error source descriptors from MM and
populates those into the HEST ACPI table. The error source descriptors
that are available from the MM side are retreived using MM Communicate 2
protocol.

The first call into the MM returns the size of MM Communicate buffer
required to hold all error source descriptor info. The communication
buffer of that size is then allocated and the second call into MM
returns the error source descriptors in the communication buffer.
The retreived error source descriptors are then appended to the HEST
table.

Co-authored-by: Thomas Abraham <thomas.abraham@arm.com>
Signed-off-by: Omkar Anand Kulkarni <omkar.kulkarni@arm.com>
---
 ArmPlatformPkg/ArmPlatformPkg.dec                                         |   7 +
 ArmPlatformPkg/Drivers/HestMmErrorSources/HestErrorSourceDxe.inf          |  41 +++
 ArmPlatformPkg/Drivers/HestMmErrorSources/HestErrorSourceStandaloneMm.inf |  51 ++++
 ArmPlatformPkg/Drivers/HestMmErrorSources/HestMmErrorSourceCommon.h       |  33 +++
 ArmPlatformPkg/Drivers/HestMmErrorSources/HestErrorSourceDxe.c            | 257 ++++++++++++++++++
 ArmPlatformPkg/Drivers/HestMmErrorSources/HestErrorSourceStandaloneMm.c   | 282 ++++++++++++++++++++
 6 files changed, 671 insertions(+)

diff --git a/ArmPlatformPkg/ArmPlatformPkg.dec b/ArmPlatformPkg/ArmPlatformPkg.dec
index 8fc44c40e813..3c12d795121e 100644
--- a/ArmPlatformPkg/ArmPlatformPkg.dec
+++ b/ArmPlatformPkg/ArmPlatformPkg.dec
@@ -35,6 +35,8 @@
 
 [Guids.common]
   gArmPlatformTokenSpaceGuid   = { 0x9c0aaed4, 0x74c5, 0x4043, { 0xb4, 0x17, 0xa3, 0x22, 0x38, 0x14, 0xce, 0x76 } }
+  gArmPlatformHestErrorSourcesGuid = { 0x76b8ab43, 0x822d, 0x4b00, { 0x9f, 0xd0, 0xf4, 0xa5, 0x35, 0x82, 0x47, 0x0a } }
+  gMmHestGetErrorSourceInfoGuid = { 0x7d602951, 0x678e, 0x4cc4, { 0x98, 0xd9, 0xe3, 0x76, 0x04, 0xf6, 0x93, 0x0d } }
 
 [PcdsFeatureFlag.common]
   gArmPlatformTokenSpaceGuid.PcdSendSgiToBringUpSecondaryCores|FALSE|BOOLEAN|0x00000004
@@ -111,6 +113,11 @@
 
   gArmPlatformTokenSpaceGuid.PcdWatchdogCount|0x0|UINT32|0x00000033
 
+[PcdsFixedAtBuild, PcdsPatchableInModule]
+  ## ACPI CPER memory space
+  gArmPlatformTokenSpaceGuid.PcdGhesGenericErrorDataMmBufferBase|0x00000000|UINT64|0x00000046
+  gArmPlatformTokenSpaceGuid.PcdGhesGenericErrorDataMmBufferSize|0x00000000|UINT64|0x00000047
+
 [Protocols.common]
   ## Arm Platform HEST table generation protocol
   gHestTableProtocolGuid = { 0x705bdcd9, 0x8c47, 0x457e, { 0xad, 0x0d, 0xf7, 0x86, 0xf3, 0x4a, 0x0d, 0x63 } }
diff --git a/ArmPlatformPkg/Drivers/HestMmErrorSources/HestErrorSourceDxe.inf b/ArmPlatformPkg/Drivers/HestMmErrorSources/HestErrorSourceDxe.inf
new file mode 100644
index 000000000000..514f60fbafd4
--- /dev/null
+++ b/ArmPlatformPkg/Drivers/HestMmErrorSources/HestErrorSourceDxe.inf
@@ -0,0 +1,41 @@
+## @file
+#  DXE driver to retrieve the error source descriptors from Standalone MM and append
+#  those to the HEST table.
+#
+#  Copyright (c) 2020, ARM Limited. All rights reserved.
+#  SPDX-License-Identifier: BSD-2-Clause-Patent
+#
+
+[Defines]
+  INF_VERSION                    = 0x0001001A
+  BASE_NAME                      = HestMmErrorSourceDxe
+  FILE_GUID                      = 76b8ab43-822d-4b00-9fd0-f4a53582470a
+  MODULE_TYPE                    = DXE_DRIVER
+  VERSION_STRING                 = 1.0
+  ENTRY_POINT                    = HestErrorSourceInitialize
+
+[Sources.common]
+  HestErrorSourceDxe.c
+
+[Packages]
+  ArmPkg/ArmPkg.dec
+  ArmPlatformPkg/ArmPlatformPkg.dec
+  MdePkg/MdePkg.dec
+  StandaloneMmPkg/StandaloneMmPkg.dec
+
+[LibraryClasses]
+  BaseMemoryLib
+  DebugLib
+  DxeServicesTableLib
+  UefiDriverEntryPoint
+  UefiLib
+
+[Guids]
+  gMmHestGetErrorSourceInfoGuid                  ## PRODUCES
+
+[Protocols]
+  gHestTableProtocolGuid                         ## CONSUMES
+  gEfiMmCommunication2ProtocolGuid
+
+[Depex]
+  gHestTableProtocolGuid AND gEfiMmCommunication2ProtocolGuid
diff --git a/ArmPlatformPkg/Drivers/HestMmErrorSources/HestErrorSourceStandaloneMm.inf b/ArmPlatformPkg/Drivers/HestMmErrorSources/HestErrorSourceStandaloneMm.inf
new file mode 100644
index 000000000000..cf215ddd8d50
--- /dev/null
+++ b/ArmPlatformPkg/Drivers/HestMmErrorSources/HestErrorSourceStandaloneMm.inf
@@ -0,0 +1,51 @@
+## @file
+#  HEST error source gateway Standalone MM driver.
+#
+#  Collects HEST error source descriptors,by communicating with all the MM drivers
+#  implementing the HEST error source descriptor protocol.
+#
+#  Copyright (c) 2020, ARM Limited. All rights reserved.
+#  SPDX-License-Identifier: BSD-2-Clause-Patent
+#
+##
+
+[Defines]
+  INF_VERSION                    = 0x0001001A
+  BASE_NAME                      = HestErrorSourceStandaloneMm
+  FILE_GUID                      = 3ddbebcc-9841-4ef8-87fa-305843c1922d
+  MODULE_TYPE                    = MM_STANDALONE
+  VERSION_STRING                 = 1.0
+  PI_SPECIFICATION_VERSION       = 0x00010032
+  ENTRY_POINT                    = StandaloneMmHestErrorSourceInitialize
+
+[Sources]
+  HestErrorSourceStandaloneMm.c
+
+[Packages]
+  ArmPkg/ArmPkg.dec
+  ArmPlatformPkg/ArmPlatformPkg.dec
+  MdeModulePkg/MdeModulePkg.dec
+  MdePkg/MdePkg.dec
+  StandaloneMmPkg/StandaloneMmPkg.dec
+
+[LibraryClasses]
+  ArmLib
+  ArmSvcLib
+  BaseMemoryLib
+  DebugLib
+  MemoryAllocationLib
+  StandaloneMmDriverEntryPoint
+
+[Protocols]
+  gMmHestErrorSourceDescProtocolGuid
+
+[Guids]
+  gMmHestGetErrorSourceInfoGuid               ##PRODUCES
+  gEfiStandaloneMmNonSecureBufferGuid
+
+[FixedPcd]
+  gArmPlatformTokenSpaceGuid.PcdGhesGenericErrorDataMmBufferBase
+  gArmPlatformTokenSpaceGuid.PcdGhesGenericErrorDataMmBufferSize
+
+[Depex]
+  gMmHestErrorSourceDescProtocolGuid
diff --git a/ArmPlatformPkg/Drivers/HestMmErrorSources/HestMmErrorSourceCommon.h b/ArmPlatformPkg/Drivers/HestMmErrorSources/HestMmErrorSourceCommon.h
new file mode 100644
index 000000000000..2929f05607ac
--- /dev/null
+++ b/ArmPlatformPkg/Drivers/HestMmErrorSources/HestMmErrorSourceCommon.h
@@ -0,0 +1,33 @@
+/** @file
+  Define data structure for error source descriptor information.
+
+  Copyright (c) 2020, ARM Limited. All rights reserved.
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+#ifndef HEST_ERROR_SOURCE_DESCRIPTOR_H_
+#define HEST_ERROR_SOURCE_DESCRIPTOR_H_
+
+#define HEST_ERROR_SOURCE_DESC_INFO_SIZE (OFFSET_OF (HEST_ERROR_SOURCE_DESC_INFO, ErrSourceDescList))
+
+//
+// Data Structure to communicate the error source descriptor information from
+// Standalone MM.
+//
+typedef struct {
+  //
+  // Total count of error source descriptors.
+  //
+  UINTN ErrSourceDescCount;
+  //
+  // Total size of all the error source descriptors.
+  //
+  UINTN ErrSourceDescSize;
+  //
+  // Array of error source descriptors that is ErrSourceDescSize in size.
+  //
+  UINT8 ErrSourceDescList[1];
+} HEST_ERROR_SOURCE_DESC_INFO;
+
+#endif // HEST_ERROR_SOURCE_DESCRIPTOR_H_
diff --git a/ArmPlatformPkg/Drivers/HestMmErrorSources/HestErrorSourceDxe.c b/ArmPlatformPkg/Drivers/HestMmErrorSources/HestErrorSourceDxe.c
new file mode 100644
index 000000000000..539424f2f4d6
--- /dev/null
+++ b/ArmPlatformPkg/Drivers/HestMmErrorSources/HestErrorSourceDxe.c
@@ -0,0 +1,257 @@
+/** @file
+  Collects and appends the HEST error source descriptors from the MM drivers.
+
+  The drivers entry point locates the MM Communication protocol and calls into
+  Standalone MM to get the HEST error sources length and count. It also
+  retrieves descriptor information.
+  The information is then used to build the HEST table using the HEST table
+  generation protocol.
+
+  Copyright (c) 2020, ARM Limited. All rights reserved.
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+#include <IndustryStandard/Acpi.h>
+
+#include <Library/BaseMemoryLib.h>
+#include <Library/DebugLib.h>
+#include <Library/DxeServicesTableLib.h>
+#include <Library/MemoryAllocationLib.h>
+#include <Library/UefiBootServicesTableLib.h>
+
+#include <Protocol/MmCommunication2.h>
+#include <Protocol/HestTable.h>
+
+#include "HestMmErrorSourceCommon.h"
+
+#define MM_COMMUNICATE_HEADER_SIZE (OFFSET_OF (EFI_MM_COMMUNICATE_HEADER, Data))
+
+STATIC HEST_TABLE_PROTOCOL *mHestProtocol;
+STATIC EFI_MM_COMMUNICATION2_PROTOCOL *mMmCommunication2;
+
+/**
+  Retrieve the error source descriptors from Standalone MM.
+
+  Initialize the MM comminication buffer by assigning the MM service to
+  invoke as gMmHestGetErrorSourceInfoGuid. Use the MM communication
+  protocol to retrieve the error source descriptors.
+
+  @param[in]      CommBuffSize  Size of communicate buffer.
+  @param[in, out] CommBuffer    The communicate buffer.
+
+  @retval EFI_SUCCESS           MM Communicate protocol call successful.
+  @retval Other                 MM Communicate protocol call failed.
+
+**/
+STATIC
+EFI_STATUS
+GetErrorSourceDescriptors (
+  IN     UINTN                     CommBuffSize,
+  IN OUT EFI_MM_COMMUNICATE_HEADER **CommBuffer
+  )
+{
+  EFI_STATUS Status;
+
+  //
+  // Initialize the CommBuffer with MM Communicate metadata.
+  //
+  CopyGuid (&(*CommBuffer)->HeaderGuid, &gMmHestGetErrorSourceInfoGuid);
+  (*CommBuffer)->MessageLength =
+    CommBuffSize -
+    sizeof ((*CommBuffer)->HeaderGuid) -
+    sizeof ((*CommBuffer)->MessageLength);
+
+  //
+  // Call into the Standalone MM using the MM Communicate protocol.
+  //
+  Status = mMmCommunication2->Communicate (
+                                mMmCommunication2,
+                                (VOID *)*CommBuffer,
+                                (VOID *)*CommBuffer,
+                                NULL
+                                );
+
+  return Status;
+}
+
+/**
+  Collect HEST error source descriptors from all Standalone MM drivers and append
+  them to the HEST table.
+
+  Use MM Communication Protocol to communicate and collect the error source
+  descriptor information from Standalone MM. Check for the required buffer size
+  returned by the MM driver. Allocate buffer of adequate size and call again into
+  MM.
+
+  @retval EFI_SUCCESS          Successful to collect and append the error source
+                               descriptors to HEST table.
+  @retval EFI_OUT_OF_RESOURCES Memory allocation failure.
+  @retval Other                For any other error.
+
+**/
+STATIC
+EFI_STATUS
+AppendMmErrorSources (VOID)
+{
+  EFI_MM_COMMUNICATE_HEADER   *CommunicationHeader = NULL;
+  HEST_ERROR_SOURCE_DESC_INFO *ErrorSourceDescInfo;
+  EFI_STATUS                  Status;
+  UINTN                       CommBufferSize;
+
+  //
+  // Find out the number of error source descriptors that StandaloneMM supports
+  // and length of the buffer to be supplied to retrieve those error source
+  // descriptors. Do this by providing a buffer of size that is just sufficient
+  // to hold the error source descriptor info. In response to this, the error
+  // source descriptor that is returned contains the actual size of the
+  // communication buffer that is required to retrieve all the error sources.
+  //
+  CommBufferSize = MM_COMMUNICATE_HEADER_SIZE + HEST_ERROR_SOURCE_DESC_INFO_SIZE;
+  CommunicationHeader = AllocatePool (CommBufferSize);
+  if (CommunicationHeader == NULL) {
+    return EFI_OUT_OF_RESOURCES;
+  }
+
+  // Call to retrieve the length of required buffer.
+  Status = GetErrorSourceDescriptors (CommBufferSize, &CommunicationHeader);
+  if ((EFI_ERROR (Status)) &&
+      (Status != EFI_BAD_BUFFER_SIZE)) {
+    DEBUG ((DEBUG_ERROR, "MM Communicate protocol call failed, status: %r\n",
+                          Status));
+    FreePool (CommunicationHeader);
+    return Status;
+  }
+
+  // Check for the length of Error Source descriptors.
+  ErrorSourceDescInfo = (HEST_ERROR_SOURCE_DESC_INFO *)(CommunicationHeader->Data);
+  if ((ErrorSourceDescInfo->ErrSourceDescSize == 0) ||
+      (ErrorSourceDescInfo->ErrSourceDescCount == 0)) {
+    DEBUG ((DEBUG_INFO, "HEST error source(s) not found\n"));
+    FreePool (CommunicationHeader);
+    return EFI_SUCCESS;
+  }
+
+  //
+  // Allocate CommBuffer of required size to accomodate all the error source
+  // descriptors. Required size of communication buffer =
+  // MM communicate metadata. + (error source desc info struct + error source
+  // descriptor size)
+  //
+  CommBufferSize =
+    MM_COMMUNICATE_HEADER_SIZE +
+    HEST_ERROR_SOURCE_DESC_INFO_SIZE +
+    ErrorSourceDescInfo->ErrSourceDescSize;
+
+  // Free old MM Communicate buffer and allocate a new buffer of required size.
+  FreePool (CommunicationHeader);
+  CommunicationHeader = AllocatePool (CommBufferSize);
+  if (CommunicationHeader == NULL) {
+    return EFI_OUT_OF_RESOURCES;
+  }
+
+  // Call to get the error source descriptors.
+  Status = GetErrorSourceDescriptors (CommBufferSize, &CommunicationHeader);
+  if (EFI_ERROR (Status)) {
+    DEBUG ((DEBUG_ERROR, "MM Communicate protocol failed, status: %r\n",
+                        Status));
+    FreePool (CommunicationHeader);
+    return Status;
+  }
+
+  //
+  // Retrieve the HEST error source descriptor information. Ensure that there
+  // is a valid list of error source descriptors.
+  //
+  ErrorSourceDescInfo = (HEST_ERROR_SOURCE_DESC_INFO *)(CommunicationHeader->Data);
+  if (ErrorSourceDescInfo->ErrSourceDescList == NULL) {
+    DEBUG ((DEBUG_INFO, "Error source descriptor list is empty"));
+    FreePool (CommunicationHeader);
+    return EFI_SUCCESS;
+  }
+
+  DEBUG ((DEBUG_INFO, "HEST ErrorSources: TotalCount = %d TotalLength = %d \n",
+                       ErrorSourceDescInfo->ErrSourceDescCount,
+                       ErrorSourceDescInfo->ErrSourceDescSize));
+  //
+  // Append the error source descriptors to HEST table using the HEST table
+  // generation protocol.
+  //
+  Status = mHestProtocol->AppendErrorSourceDescriptors (
+                            ErrorSourceDescInfo->ErrSourceDescList,
+                            ErrorSourceDescInfo->ErrSourceDescSize,
+                            ErrorSourceDescInfo->ErrSourceDescCount
+                            );
+  if (EFI_ERROR (Status)) {
+    DEBUG ((DEBUG_ERROR, "Failed to append error source(s), status: %r\n",
+                         Status));
+  }
+
+  FreePool (CommunicationHeader);
+  return Status;
+}
+
+/**
+  The Entry Point for Hest Error Source DXE driver.
+
+  Locates the Hest Table generation and MM Communication2 protocols. Using the
+  MM Communication2, the driver collects the Error Source Descriptor(s) from
+  Standalone MM. It then appends those Error Source Descriptor(s) to the Hest
+  table using the Hest Table generation protocol.
+
+  @param[in] ImageHandle The firmware allocated handle for the EFI image.
+  @param[in] SystemTable A pointer to the EFI System Table.
+
+  @retval EFI_SUCCESS    The entry point is executed successfully.
+  @retval Other          Some error occurred when executing this entry point.
+
+**/
+EFI_STATUS
+EFIAPI
+HestErrorSourceInitialize (
+  IN EFI_HANDLE       ImageHandle,
+  IN EFI_SYSTEM_TABLE *SystemTable
+  )
+{
+  EFI_STATUS Status;
+
+  Status = gBS->LocateProtocol (
+                  &gHestTableProtocolGuid,
+                  NULL,
+                  (VOID **)&mHestProtocol
+                  );
+  if (EFI_ERROR (Status)) {
+    DEBUG ((
+      DEBUG_ERROR,
+      "Failed to locate HEST table generation protocol, status:%r\n",
+      Status
+      ));
+    return Status;
+  }
+
+  Status = gBS->LocateProtocol (
+                  &gEfiMmCommunication2ProtocolGuid,
+                  NULL,
+                  (VOID **)&mMmCommunication2
+                  );
+  if (EFI_ERROR (Status)) {
+    DEBUG ((
+      DEBUG_ERROR,
+      "Failed to locate MMCommunication2 driver protocol, status:%r\n",
+      Status
+      ));
+    return Status;
+  }
+
+  //
+  // Append HEST error sources retrieved from StandaloneMM, if any, into the HEST
+  // ACPI table.
+  //
+  Status = AppendMmErrorSources ();
+  if (EFI_ERROR (Status)) {
+    DEBUG ((DEBUG_ERROR, "HEST table creation faied, status:%r\n",
+                         Status));
+  }
+
+  return EFI_SUCCESS;
+}
diff --git a/ArmPlatformPkg/Drivers/HestMmErrorSources/HestErrorSourceStandaloneMm.c b/ArmPlatformPkg/Drivers/HestMmErrorSources/HestErrorSourceStandaloneMm.c
new file mode 100644
index 000000000000..9d0c1f744579
--- /dev/null
+++ b/ArmPlatformPkg/Drivers/HestMmErrorSources/HestErrorSourceStandaloneMm.c
@@ -0,0 +1,282 @@
+/** @file
+  MM HEST error source gateway driver.
+
+  This driver installs a handler which can be used to retrieve the error source
+  descriptors from the all MM drivers implementing the HEST error source
+  descriptor protocol.
+
+  Copyright (c) 2020, ARM Limited. All rights reserved.
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+#include <Base.h>
+
+#include <Library/BaseLib.h>
+#include <Library/BaseMemoryLib.h>
+#include <Library/DebugLib.h>
+#include <Library/MemoryAllocationLib.h>
+
+#include <Protocol/HestErrorSourceInfo.h>
+#include "HestMmErrorSourceCommon.h"
+
+STATIC EFI_MM_SYSTEM_TABLE *mMmst = NULL;
+
+/**
+  Returns an array of handles that implement the HEST error source descriptor
+  protocol.
+
+  Passing HandleBuffer as NULL will return the actual size of the buffer
+  required to hold the array of handles implementing the protocol.
+
+  @param[in, out] HandleBufferSize  The size of the HandleBuffer.
+  @param[out]     HandleBuffer      A pointer to the buffer containing the list
+                                    of handles.
+
+  @retval EFI_SUCCESS           The array of handles returned in HandleBuffer.
+  @retval EFI_NOT_FOUND         No implementation present for the protocol.
+  @retval Other                 For any other error.
+
+**/
+STATIC
+EFI_STATUS
+GetHestErrorSourceProtocolHandles (
+  IN OUT UINTN      *HandleBufferSize,
+  OUT    EFI_HANDLE **HandleBuffer
+  )
+{
+  EFI_STATUS Status;
+
+  Status = mMmst->MmLocateHandle (
+                    ByProtocol,
+                    &gMmHestErrorSourceDescProtocolGuid,
+                    NULL,
+                    HandleBufferSize,
+                    *HandleBuffer
+                    );
+  if ((EFI_ERROR (Status)) &&
+      (Status != EFI_BUFFER_TOO_SMALL))
+  {
+    DEBUG ((
+      DEBUG_ERROR,
+      "No implementation of MmHestErrorSourceDescProtocol found, Status:%r\n",
+      Status
+      ));
+    return EFI_NOT_FOUND;
+  }
+
+  return Status;
+}
+
+/**
+  MMI handler to retrieve HEST error source descriptor information.
+
+  Handler for MMI service that returns the supported HEST error source
+  descriptors in MM. This handler populates the CommBuffer with the
+  list of all error source descriptors, prepended with the length and
+  the number of descriptors populated into CommBuffer.
+
+  @param[in]      DispatchHandle The unique handle assigned to this handler by
+                                 MmiHandlerRegister().
+  @param[in]      Context        Points to an optional handler context which was
+                                 specified when the handler was registered.
+  @param[in, out] CommBuffer     Buffer  used for communication of HEST error
+                                 source descriptors.
+  @param[in, out] CommBufferSize The size of the CommBuffer.
+
+  @return EFI_SUCCESS           CommBuffer has valid data.
+  @return EFI_BAD_BUFFER_SIZE   CommBufferSize not adequate.
+  @return EFI_OUT_OF_RESOURCES  System out of memory resources.
+  @retval EFI_INVALID_PARAMETER Invalid CommBufferSize recieved.
+  @retval Other                 For any other error.
+
+**/
+STATIC
+EFI_STATUS
+EFIAPI
+HestErrorSourcesInfoMmiHandler (
+  IN     EFI_HANDLE DispatchHandle,
+  IN     CONST VOID *Context,       OPTIONAL
+  IN OUT VOID       *CommBuffer,    OPTIONAL
+  IN OUT UINTN      *CommBufferSize OPTIONAL
+  )
+{
+  MM_HEST_ERROR_SOURCE_DESC_PROTOCOL *HestErrSourceDescProtocolHandle;
+  HEST_ERROR_SOURCE_DESC_INFO        *ErrorSourceInfoList;
+  EFI_HANDLE                         *HandleBuffer;
+  EFI_STATUS                         Status;
+  UINTN                              HandleCount;
+  UINTN                              HandleBufferSize;
+  UINTN                              Index;
+  UINTN                              SourceCount = 0;
+  UINTN                              SourceLength = 0;
+  VOID                               *ErrorSourcePtr;
+  UINTN                              TotalSourceLength = 0;
+  UINTN                              TotalSourceCount = 0;
+
+  if (*CommBufferSize < HEST_ERROR_SOURCE_DESC_INFO_SIZE) {
+    //
+    // Ensure that the communication buffer has enough space to hold the
+    // ErrSourceDescCount and ErrSourceDescSize elements of the
+    // HEST_ERROR_SOURCE_DESC_INFO structure
+    //
+    return EFI_INVALID_PARAMETER;
+  }
+
+  // Get all handles that implement the HEST error source descriptor protocol.
+
+  // Get the buffer size required to store list of handles for the protocol.
+  HandleBuffer = NULL;
+  HandleBufferSize = 0;
+  Status = GetHestErrorSourceProtocolHandles (&HandleBufferSize, &HandleBuffer);
+  if ((Status == EFI_NOT_FOUND) ||
+      (HandleBufferSize == 0))
+  {
+    return Status;
+  }
+
+  // Allocate memory for HandleBuffer of size HandleBufferSize.
+  HandleBuffer = AllocatePool (HandleBufferSize);
+  if (HandleBuffer == NULL) {
+    return EFI_OUT_OF_RESOURCES;
+  }
+
+  // Get the list of handles.
+  Status = GetHestErrorSourceProtocolHandles (&HandleBufferSize, &HandleBuffer);
+  if ((EFI_ERROR (Status)) ||
+      (HandleBuffer == NULL))
+  {
+    FreePool (HandleBuffer);
+    return Status;
+  }
+
+  // Count of handles for the protocol.
+  HandleCount = HandleBufferSize / sizeof (EFI_HANDLE);
+
+  //
+  // Loop to get the count and length of the error source descriptors from all
+  // the MM drivers implementing HEST error source descriptor protocol.
+  //
+  for (Index = 0; Index < HandleCount; ++Index) {
+    Status = mMmst->MmHandleProtocol (
+                      HandleBuffer[Index],
+                      &gMmHestErrorSourceDescProtocolGuid,
+                      (VOID **)&HestErrSourceDescProtocolHandle
+                      );
+    if (EFI_ERROR (Status)) {
+      continue;
+    }
+
+    //
+    // Protocol called with Buffer parameter passed as NULL, must return
+    // error source length and error count for that driver.
+    //
+    Status = HestErrSourceDescProtocolHandle->GetHestErrorSourceDescriptors (
+                                                HestErrSourceDescProtocolHandle,
+                                                NULL,
+                                                &SourceLength,
+                                                &SourceCount
+                                                );
+    if (Status == EFI_INVALID_PARAMETER) {
+      TotalSourceLength += SourceLength;
+      TotalSourceCount += SourceCount;
+    }
+  }
+
+  // Set the count and length in the error source descriptor.
+  ErrorSourceInfoList = (HEST_ERROR_SOURCE_DESC_INFO *)(CommBuffer);
+  ErrorSourceInfoList->ErrSourceDescCount = TotalSourceCount;
+  ErrorSourceInfoList->ErrSourceDescSize = TotalSourceLength;
+
+  //
+  // Check the size of CommBuffer, it should atleast be of size
+  // TotalSourceLength + HEST_ERROR_SOURCE_DESC_INFO_SIZE.
+  //
+  TotalSourceLength = TotalSourceLength + HEST_ERROR_SOURCE_DESC_INFO_SIZE;
+  if ((*CommBufferSize) < TotalSourceLength) {
+    FreePool (HandleBuffer);
+    return EFI_BAD_BUFFER_SIZE;
+  }
+
+  //
+  // CommBuffer size is adequate to return all the error source descriptors.
+  // Populate it with the error source descriptor information.
+  //
+
+  // Buffer pointer to append the Error Descriptors data.
+  ErrorSourcePtr =  ErrorSourceInfoList->ErrSourceDescList;
+
+  // Loop to retrieve error source descriptors.
+  for (Index = 0; Index < HandleCount; ++Index) {
+    Status = mMmst->MmHandleProtocol (
+                      HandleBuffer[Index],
+                      &gMmHestErrorSourceDescProtocolGuid,
+                      (VOID **)&HestErrSourceDescProtocolHandle
+                      );
+    if (EFI_ERROR (Status)) {
+      continue;
+    }
+
+    Status = HestErrSourceDescProtocolHandle->GetHestErrorSourceDescriptors (
+                                                HestErrSourceDescProtocolHandle,
+                                                (VOID **)&ErrorSourcePtr,
+                                                &SourceLength,
+                                                &SourceCount
+                                                );
+    if (Status == EFI_SUCCESS) {
+      ErrorSourcePtr += SourceLength;
+    }
+  }
+
+  // Free the buffer holding all the protocol handles.
+  FreePool (HandleBuffer);
+
+  // Initialize CPER memory.
+  SetMem (
+    (VOID *)FixedPcdGet64 (PcdGhesGenericErrorDataMmBufferBase),
+    FixedPcdGet64 (PcdGhesGenericErrorDataMmBufferSize),
+    0
+    );
+
+  return EFI_SUCCESS;
+}
+
+/**
+  Entry point for this Stanalone MM driver.
+
+  Registers an MMI handler that retrieves the error source descriptors from all
+  the MM drivers implementing the MM_HEST_ERROR_SOURCE_DESC_PROTOCOL.
+
+  @param[in] ImageHandle  The firmware allocated handle for the EFI image.
+  @param[in] SystemTable  A pointer to the EFI System Table.
+
+  @retval EFI_SUCCESS     The entry point registered handler successfully.
+  @retval Other           Some error occurred when executing this entry point.
+
+**/
+EFI_STATUS
+EFIAPI
+StandaloneMmHestErrorSourceInitialize (
+  IN EFI_HANDLE          ImageHandle,
+  IN EFI_MM_SYSTEM_TABLE *SystemTable
+  )
+{
+  EFI_HANDLE DispatchHandle;
+  EFI_STATUS Status;
+
+  ASSERT (SystemTable != NULL);
+  mMmst = SystemTable;
+
+  Status = mMmst->MmiHandlerRegister (
+                    HestErrorSourcesInfoMmiHandler,
+                    &gMmHestGetErrorSourceInfoGuid,
+                    &DispatchHandle
+                    );
+  if (EFI_ERROR(Status)) {
+    DEBUG ((DEBUG_ERROR, "MMI handler registration failed with status : %r\n",
+                         Status));
+    return Status;
+  }
+
+  return EFI_SUCCESS;
+}
-- 
2.17.1