CpuHotplug.c

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#include <CpuHotPlugData.h>                  // CPU_HOT_PLUG_DATA
#include <IndustryStandard/Q35MchIch9.h>     // ICH9_APM_CNT
#include <IndustryStandard/QemuCpuHotplug.h> // QEMU_CPUHP_CMD_GET_PENDING
#include <Library/BaseLib.h>                 // CpuDeadLoop()
#include <Library/CpuLib.h>                  // CpuSleep()
#include <Library/DebugLib.h>                // ASSERT()
#include <Library/MmServicesTableLib.h>      // gMmst
#include <Library/PcdLib.h>                  // PcdGetBool()
#include <Library/SafeIntLib.h>              // SafeUintnSub()
#include <Pcd/CpuHotEjectData.h>             // CPU_HOT_EJECT_DATA
#include <Protocol/MmCpuIo.h>                // EFI_MM_CPU_IO_PROTOCOL
#include <Protocol/SmmCpuService.h>          // EFI_SMM_CPU_SERVICE_PROTOCOL
#include <Register/Intel/ArchitecturalMsr.h> // MSR_IA32_APIC_BASE_REGISTER
#include <Uefi/UefiBaseType.h>               // EFI_STATUS
 
#include "ApicId.h"                          // APIC_ID
#include "QemuCpuhp.h"                       // QemuCpuhpWriteCpuSelector()
#include "Smbase.h"                          // SmbaseAllocatePostSmmPen()
 
//
// We use this protocol for accessing IO Ports.
//
STATIC EFI_MM_CPU_IO_PROTOCOL  *mMmCpuIo;
//
// The following protocol is used to report the addition or removal of a CPU to
// the SMM CPU driver (PiSmmCpuDxeSmm).
//
STATIC EFI_SMM_CPU_SERVICE_PROTOCOL  *mMmCpuService;
//
// These structures serve as communication side-channels between the
// EFI_SMM_CPU_SERVICE_PROTOCOL consumer (i.e., this driver) and provider
// (i.e., PiSmmCpuDxeSmm).
//
STATIC CPU_HOT_PLUG_DATA   *mCpuHotPlugData;
STATIC CPU_HOT_EJECT_DATA  *mCpuHotEjectData;
//
// SMRAM arrays for fetching the APIC IDs of processors with pending events (of
// known event types), for the time of just one MMI.
//
// The lifetimes of these arrays match that of this driver only because we
// don't want to allocate SMRAM at OS runtime, and potentially fail (or
// fragment the SMRAM map).
//
// The first array stores APIC IDs for hot-plug events, the second and the
// third store APIC IDs and QEMU CPU Selectors (both indexed similarly) for
// hot-unplug events. All of these provide room for "possible CPU count" minus
// one elements as we don't expect every possible CPU to appear, or disappear,
// in a single MMI. The numbers of used (populated) elements in the arrays are
// determined on every MMI separately.
//
STATIC APIC_ID  *mPluggedApicIds;
STATIC APIC_ID  *mToUnplugApicIds;
STATIC UINT32   *mToUnplugSelectors;
//
// Address of the non-SMRAM reserved memory page that contains the Post-SMM Pen
// for hot-added CPUs.
//
STATIC UINT32  mPostSmmPenAddress;
//
// Represents the registration of the CPU Hotplug MMI handler.
//
STATIC EFI_HANDLE  mDispatchHandle;
 
STATIC
EFI_STATUS
ProcessHotAddedCpus (
  IN APIC_ID  *PluggedApicIds,
  IN UINT32   PluggedCount
  )
{
  EFI_STATUS  Status;
  UINT32      PluggedIdx;
  UINT32      NewSlot;
 
  //
  // The Post-SMM Pen need not be reinstalled multiple times within a single
  // root MMI handling. Even reinstalling once per root MMI is only prudence;
  // in theory installing the pen in the driver's entry point function should
  // suffice.
  //
  SmbaseReinstallPostSmmPen (mPostSmmPenAddress);
 
  PluggedIdx = 0;
  NewSlot    = 0;
  while (PluggedIdx < PluggedCount) {
    APIC_ID  NewApicId;
    UINT32   CheckSlot;
    UINTN    NewProcessorNumberByProtocol;
 
    NewApicId = PluggedApicIds[PluggedIdx];
 
    //
    // Check if the supposedly hot-added CPU is already known to us.
    //
    for (CheckSlot = 0;
         CheckSlot < mCpuHotPlugData->ArrayLength;
         CheckSlot++)
    {
      if (mCpuHotPlugData->ApicId[CheckSlot] == NewApicId) {
        break;
      }
    }
 
    if (CheckSlot < mCpuHotPlugData->ArrayLength) {
      DEBUG ((
        DEBUG_VERBOSE,
        "%a: APIC ID " FMT_APIC_ID " was hot-plugged "
                                   "before; ignoring it\n",
        __func__,
        NewApicId
        ));
      PluggedIdx++;
      continue;
    }
 
    //
    // Find the first empty slot in CPU_HOT_PLUG_DATA.
    //
    while (NewSlot < mCpuHotPlugData->ArrayLength &&
           mCpuHotPlugData->ApicId[NewSlot] != MAX_UINT64)
    {
      NewSlot++;
    }
 
    if (NewSlot == mCpuHotPlugData->ArrayLength) {
      DEBUG ((
        DEBUG_ERROR,
        "%a: no room for APIC ID " FMT_APIC_ID "\n",
        __func__,
        NewApicId
        ));
      return EFI_OUT_OF_RESOURCES;
    }
 
    //
    // Store the APIC ID of the new processor to the slot.
    //
    mCpuHotPlugData->ApicId[NewSlot] = NewApicId;
 
    //
    // Relocate the SMBASE of the new CPU.
    //
    Status = SmbaseRelocate (
               NewApicId,
               mCpuHotPlugData->SmBase[NewSlot],
               mPostSmmPenAddress
               );
    if (EFI_ERROR (Status)) {
      goto RevokeNewSlot;
    }
 
    //
    // Add the new CPU with EFI_SMM_CPU_SERVICE_PROTOCOL.
    //
    Status = mMmCpuService->AddProcessor (
                              mMmCpuService,
                              NewApicId,
                              &NewProcessorNumberByProtocol
                              );
    if (EFI_ERROR (Status)) {
      DEBUG ((
        DEBUG_ERROR,
        "%a: AddProcessor(" FMT_APIC_ID "): %r\n",
        __func__,
        NewApicId,
        Status
        ));
      goto RevokeNewSlot;
    }
 
    DEBUG ((
      DEBUG_INFO,
      "%a: hot-added APIC ID " FMT_APIC_ID ", SMBASE 0x%Lx, "
                                           "EFI_SMM_CPU_SERVICE_PROTOCOL assigned number %Lu\n",
      __func__,
      NewApicId,
      (UINT64)mCpuHotPlugData->SmBase[NewSlot],
      (UINT64)NewProcessorNumberByProtocol
      ));
 
    NewSlot++;
    PluggedIdx++;
  }
 
  //
  // We've processed this batch of hot-added CPUs.
  //
  return EFI_SUCCESS;
 
RevokeNewSlot:
  mCpuHotPlugData->ApicId[NewSlot] = MAX_UINT64;
 
  return Status;
}
 
STATIC
BOOLEAN
CheckIfBsp (
  VOID
  )
{
  MSR_IA32_APIC_BASE_REGISTER  ApicBaseMsr;
  BOOLEAN                      IsBsp;
 
  ApicBaseMsr.Uint64 = AsmReadMsr64 (MSR_IA32_APIC_BASE);
  IsBsp              = (BOOLEAN)(ApicBaseMsr.Bits.BSP == 1);
  return IsBsp;
}
 
VOID
EFIAPI
EjectCpu (
  IN UINTN  ProcessorNum
  )
{
  UINT64  QemuSelector;
 
  if (CheckIfBsp ()) {
    UINT32  Idx;
 
    for (Idx = 0; Idx < mCpuHotEjectData->ArrayLength; Idx++) {
      QemuSelector = mCpuHotEjectData->QemuSelectorMap[Idx];
 
      if (QemuSelector != CPU_EJECT_QEMU_SELECTOR_INVALID) {
        //
        // This to-be-ejected-CPU has already received the BSP's SMI exit
        // signal and will execute SmmCpuFeaturesRendezvousExit()
        // followed by this callback or is already penned in the
        // CpuSleep() loop below.
        //
        // Tell QEMU to context-switch it out.
        //
        QemuCpuhpWriteCpuSelector (mMmCpuIo, (UINT32)QemuSelector);
        QemuCpuhpWriteCpuStatus (mMmCpuIo, QEMU_CPUHP_STAT_EJECT);
 
        //
        // Now that we've ejected the CPU corresponding to QemuSelectorMap[Idx],
        // clear its eject status to ensure that an invalid future SMI does
        // not end up trying a spurious eject or a newly hotplugged CPU does
        // not get penned in the CpuSleep() loop.
        //
        // Note that the QemuCpuhpWriteCpuStatus() command above is a write to
        // a different address space and uses the EFI_MM_CPU_IO_PROTOCOL.
        //
        // This means that we are guaranteed that the following assignment
        // will not be reordered before the eject. And, so we can safely
        // do this write here.
        //
        mCpuHotEjectData->QemuSelectorMap[Idx] =
          CPU_EJECT_QEMU_SELECTOR_INVALID;
 
        DEBUG ((
          DEBUG_INFO,
          "%a: Unplugged ProcessorNum %u, "
          "QemuSelector %Lu\n",
          __func__,
          Idx,
          QemuSelector
          ));
      }
    }
 
    //
    // We are done until the next hot-unplug; clear the handler.
    //
    // mCpuHotEjectData->Handler is a NOP for any CPU not under ejection.
    // So, once we are done with all the ejections, we can safely reset it
    // here since any CPU dereferencing it would only see either the old
    // or the new value (since it is aligned at a natural boundary.)
    //
    mCpuHotEjectData->Handler = NULL;
    return;
  }
 
  //
  // Reached only on APs
  //
 
  //
  // mCpuHotEjectData->QemuSelectorMap[ProcessorNum] is updated
  // on the BSP in the ongoing SMI at two places:
  //
  // - UnplugCpus() where the BSP determines if a CPU is under ejection
  //   or not. As a comment in UnplugCpus() at set-up, and in
  //   SmmCpuFeaturesRendezvousExit() where it is dereferenced describe,
  //   any such updates are guaranteed to be ordered-before the
  //   dereference below.
  //
  // - EjectCpu() on the BSP (above) updates QemuSelectorMap[ProcessorNum]
  //   for a CPU once it's ejected.
  //
  //   The CPU under ejection: might be executing anywhere between the
  //   AllCpusInSync loop in SmiRendezvous(), to about to dereference
  //   QemuSelectorMap[ProcessorNum].
  //   As described in the comment above where we do the reset, this
  //   is not a problem since the ejected CPU never sees the after value.
  //   CPUs not-under ejection: never see any changes so they are fine.
  //
  QemuSelector = mCpuHotEjectData->QemuSelectorMap[ProcessorNum];
  if (QemuSelector == CPU_EJECT_QEMU_SELECTOR_INVALID) {
    /* wait until BSP is done */
    while (mCpuHotEjectData->Handler != NULL) {
      CpuPause ();
    }
 
    return;
  }
 
  //
  // APs being unplugged get here from SmmCpuFeaturesRendezvousExit()
  // after having been cleared to exit the SMI and so have no SMM
  // processing remaining.
  //
  // Keep them penned here until the BSP tells QEMU to eject them.
  //
  for ( ; ;) {
    DisableInterrupts ();
    CpuSleep ();
  }
}
 
STATIC
EFI_STATUS
UnplugCpus (
  IN APIC_ID  *ToUnplugApicIds,
  IN UINT32   *ToUnplugSelectors,
  IN UINT32   ToUnplugCount
  )
{
  EFI_STATUS  Status;
  UINT32      ToUnplugIdx;
  UINT32      EjectCount;
  UINTN       ProcessorNum;
 
  ToUnplugIdx = 0;
  EjectCount  = 0;
  while (ToUnplugIdx < ToUnplugCount) {
    APIC_ID  RemoveApicId;
    UINT32   QemuSelector;
 
    RemoveApicId = ToUnplugApicIds[ToUnplugIdx];
    QemuSelector = ToUnplugSelectors[ToUnplugIdx];
 
    //
    // mCpuHotPlugData->ApicId maps ProcessorNum -> ApicId. Use RemoveApicId
    // to find the corresponding ProcessorNum for the CPU to be removed.
    //
    // With this we can establish a 3 way mapping:
    //    APIC_ID -- ProcessorNum -- QemuSelector
    //
    // We stash the ProcessorNum -> QemuSelector mapping so it can later be
    // used for CPU hot-eject in SmmCpuFeaturesRendezvousExit() context (where
    // we only have ProcessorNum available.)
    //
 
    for (ProcessorNum = 0;
         ProcessorNum < mCpuHotPlugData->ArrayLength;
         ProcessorNum++)
    {
      if (mCpuHotPlugData->ApicId[ProcessorNum] == RemoveApicId) {
        break;
      }
    }
 
    //
    // Ignore the unplug if APIC ID not found
    //
    if (ProcessorNum == mCpuHotPlugData->ArrayLength) {
      DEBUG ((
        DEBUG_VERBOSE,
        "%a: did not find APIC ID " FMT_APIC_ID
        " to unplug\n",
        __func__,
        RemoveApicId
        ));
      ToUnplugIdx++;
      continue;
    }
 
    //
    // Mark ProcessorNum for removal from SMM data structures
    //
    Status = mMmCpuService->RemoveProcessor (mMmCpuService, ProcessorNum);
    if (EFI_ERROR (Status)) {
      DEBUG ((
        DEBUG_ERROR,
        "%a: RemoveProcessor(" FMT_APIC_ID "): %r\n",
        __func__,
        RemoveApicId,
        Status
        ));
      return Status;
    }
 
    if (mCpuHotEjectData->QemuSelectorMap[ProcessorNum] !=
        CPU_EJECT_QEMU_SELECTOR_INVALID)
    {
      //
      // mCpuHotEjectData->QemuSelectorMap[ProcessorNum] is set to
      // CPU_EJECT_QEMU_SELECTOR_INVALID when mCpuHotEjectData->QemuSelectorMap
      // is allocated, and once the subject processsor is ejected.
      //
      // Additionally, mMmCpuService->RemoveProcessor(ProcessorNum) invalidates
      // mCpuHotPlugData->ApicId[ProcessorNum], so a given ProcessorNum can
      // never match more than one APIC ID -- nor, by transitivity, designate
      // more than one QemuSelector -- in a single invocation of UnplugCpus().
      //
      DEBUG ((
        DEBUG_ERROR,
        "%a: ProcessorNum %Lu maps to QemuSelector %Lu, "
        "cannot also map to %u\n",
        __func__,
        (UINT64)ProcessorNum,
        mCpuHotEjectData->QemuSelectorMap[ProcessorNum],
        QemuSelector
        ));
 
      return EFI_ALREADY_STARTED;
    }
 
    //
    // Stash the QemuSelector so we can do the actual ejection later.
    //
    mCpuHotEjectData->QemuSelectorMap[ProcessorNum] = (UINT64)QemuSelector;
 
    DEBUG ((
      DEBUG_INFO,
      "%a: Started hot-unplug on ProcessorNum %Lu, APIC ID "
      FMT_APIC_ID ", QemuSelector %u\n",
      __func__,
      (UINT64)ProcessorNum,
      RemoveApicId,
      QemuSelector
      ));
 
    EjectCount++;
    ToUnplugIdx++;
  }
 
  if (EjectCount != 0) {
    //
    // We have processors to be ejected; install the handler.
    //
    mCpuHotEjectData->Handler = EjectCpu;
 
    //
    // The BSP and APs load mCpuHotEjectData->Handler, and
    // mCpuHotEjectData->QemuSelectorMap[] in SmmCpuFeaturesRendezvousExit()
    // and EjectCpu().
    //
    // The comment in SmmCpuFeaturesRendezvousExit() details how we use
    // the AllCpusInSync control-dependency to ensure that any loads are
    // ordered-after the stores above.
    //
    // Ensure that the stores above are ordered-before the AllCpusInSync store
    // by using a MemoryFence() with release semantics.
    //
    MemoryFence ();
  }
 
  //
  // We've removed this set of APIC IDs from SMM data structures and
  // have installed an ejection handler if needed.
  //
  return EFI_SUCCESS;
}
 
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;
  UINT32      PluggedCount;
  UINT32      ToUnplugCount;
 
  //
  // 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",
      __func__,
      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;
  }
 
  //
  // Collect the CPUs with pending events.
  //
  Status = QemuCpuhpCollectApicIds (
             mMmCpuIo,
             mCpuHotPlugData->ArrayLength,     // PossibleCpuCount
             mCpuHotPlugData->ArrayLength - 1, // ApicIdCount
             mPluggedApicIds,
             &PluggedCount,
             mToUnplugApicIds,
             mToUnplugSelectors,
             &ToUnplugCount
             );
  if (EFI_ERROR (Status)) {
    goto Fatal;
  }
 
  if (PluggedCount > 0) {
    Status = ProcessHotAddedCpus (mPluggedApicIds, PluggedCount);
    if (EFI_ERROR (Status)) {
      goto Fatal;
    }
  }
 
  if (ToUnplugCount > 0) {
    Status = UnplugCpus (mToUnplugApicIds, mToUnplugSelectors, ToUnplugCount);
    if (EFI_ERROR (Status)) {
      goto Fatal;
    }
  }
 
  //
  // 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;
  UINTN       Len;
  UINTN       Size;
  UINTN       SizeSel;
 
  //
  // 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", __func__, Status));
    goto Fatal;
  }
 
  Status = gMmst->MmLocateProtocol (
                    &gEfiSmmCpuServiceProtocolGuid,
                    NULL /* Registration */,
                    (VOID **)&mMmCpuService
                    );
  if (EFI_ERROR (Status)) {
    DEBUG ((
      DEBUG_ERROR,
      "%a: locate MmCpuService: %r\n",
      __func__,
      Status
      ));
    goto Fatal;
  }
 
  //
  // Our DEPEX on EFI_SMM_CPU_SERVICE_PROTOCOL guarantees that PiSmmCpuDxeSmm
  // has pointed:
  // - PcdCpuHotPlugDataAddress to CPU_HOT_PLUG_DATA in SMRAM,
  // - PcdCpuHotEjectDataAddress to CPU_HOT_EJECT_DATA in SMRAM, if the
  //   possible CPU count is greater than 1.
  //
  mCpuHotPlugData  = (VOID *)(UINTN)PcdGet64 (PcdCpuHotPlugDataAddress);
  mCpuHotEjectData = (VOID *)(UINTN)PcdGet64 (PcdCpuHotEjectDataAddress);
 
  if (mCpuHotPlugData == NULL) {
    Status = EFI_NOT_FOUND;
    DEBUG ((DEBUG_ERROR, "%a: CPU_HOT_PLUG_DATA: %r\n", __func__, Status));
    goto Fatal;
  }
 
  //
  // If the possible CPU count is 1, there's nothing for this driver to do.
  //
  if (mCpuHotPlugData->ArrayLength == 1) {
    return EFI_UNSUPPORTED;
  }
 
  if (mCpuHotEjectData == NULL) {
    Status = EFI_NOT_FOUND;
  } else if (mCpuHotPlugData->ArrayLength != mCpuHotEjectData->ArrayLength) {
    Status = EFI_INVALID_PARAMETER;
  } else {
    Status = EFI_SUCCESS;
  }
 
  if (EFI_ERROR (Status)) {
    DEBUG ((DEBUG_ERROR, "%a: CPU_HOT_EJECT_DATA: %r\n", __func__, Status));
    goto Fatal;
  }
 
  //
  // Allocate the data structures that depend on the possible CPU count.
  //
  if (RETURN_ERROR (SafeUintnSub (mCpuHotPlugData->ArrayLength, 1, &Len)) ||
      RETURN_ERROR (SafeUintnMult (sizeof (APIC_ID), Len, &Size)) ||
      RETURN_ERROR (SafeUintnMult (sizeof (UINT32), Len, &SizeSel)))
  {
    Status = EFI_ABORTED;
    DEBUG ((DEBUG_ERROR, "%a: invalid CPU_HOT_PLUG_DATA\n", __func__));
    goto Fatal;
  }
 
  Status = gMmst->MmAllocatePool (
                    EfiRuntimeServicesData,
                    Size,
                    (VOID **)&mPluggedApicIds
                    );
  if (EFI_ERROR (Status)) {
    DEBUG ((DEBUG_ERROR, "%a: MmAllocatePool(): %r\n", __func__, Status));
    goto Fatal;
  }
 
  Status = gMmst->MmAllocatePool (
                    EfiRuntimeServicesData,
                    Size,
                    (VOID **)&mToUnplugApicIds
                    );
  if (EFI_ERROR (Status)) {
    DEBUG ((DEBUG_ERROR, "%a: MmAllocatePool(): %r\n", __func__, Status));
    goto ReleasePluggedApicIds;
  }
 
  Status = gMmst->MmAllocatePool (
                    EfiRuntimeServicesData,
                    SizeSel,
                    (VOID **)&mToUnplugSelectors
                    );
  if (EFI_ERROR (Status)) {
    DEBUG ((DEBUG_ERROR, "%a: MmAllocatePool(): %r\n", __func__, Status));
    goto ReleaseToUnplugApicIds;
  }
 
  //
  // Allocate the Post-SMM Pen for hot-added CPUs.
  //
  Status = SmbaseAllocatePostSmmPen (
             &mPostSmmPenAddress,
             SystemTable->BootServices
             );
  if (EFI_ERROR (Status)) {
    goto ReleaseToUnplugSelectors;
  }
 
  //
  // Sanity-check the CPU hotplug interface.
  //
  // Both of the following features are part of QEMU 5.0, introduced primarily
  // in commit range 3e08b2b9cb64..3a61c8db9d25:
  //
  // (a) the QEMU_CPUHP_CMD_GET_ARCH_ID command of the modern CPU hotplug
  //     interface,
  //
  // (b) the "SMRAM at default SMBASE" feature.
  //
  // From these, (b) is restricted to 5.0+ machine type versions, while (a)
  // does not depend on machine type version. Because we ensured the stricter
  // condition (b) through PcdQ35SmramAtDefaultSmbase above, the (a)
  // QEMU_CPUHP_CMD_GET_ARCH_ID command must now be available too. While we
  // can't verify the presence of precisely that command, we can still verify
  // (sanity-check) that the modern interface is active, at least.
  //
  // Consult the "Typical usecases | Detecting and enabling modern CPU hotplug
  // interface" section in QEMU's "docs/specs/acpi_cpu_hotplug.txt", on the
  // following.
  //
  QemuCpuhpWriteCpuSelector (mMmCpuIo, 0);
  QemuCpuhpWriteCpuSelector (mMmCpuIo, 0);
  QemuCpuhpWriteCommand (mMmCpuIo, QEMU_CPUHP_CMD_GET_PENDING);
  if (QemuCpuhpReadCommandData2 (mMmCpuIo) != 0) {
    Status = EFI_NOT_FOUND;
    DEBUG ((
      DEBUG_ERROR,
      "%a: modern CPU hotplug interface: %r\n",
      __func__,
      Status
      ));
    goto ReleasePostSmmPen;
  }
 
  //
  // 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",
      __func__,
      Status
      ));
    goto ReleasePostSmmPen;
  }
 
  //
  // Install the handler for the hot-added CPUs' first SMI.
  //
  SmbaseInstallFirstSmiHandler ();
 
  return EFI_SUCCESS;
 
ReleasePostSmmPen:
  SmbaseReleasePostSmmPen (mPostSmmPenAddress, SystemTable->BootServices);
  mPostSmmPenAddress = 0;
 
ReleaseToUnplugSelectors:
  gMmst->MmFreePool (mToUnplugSelectors);
  mToUnplugSelectors = NULL;
 
ReleaseToUnplugApicIds:
  gMmst->MmFreePool (mToUnplugApicIds);
  mToUnplugApicIds = NULL;
 
ReleasePluggedApicIds:
  gMmst->MmFreePool (mPluggedApicIds);
  mPluggedApicIds = NULL;
 
Fatal:
  ASSERT (FALSE);
  CpuDeadLoop ();
  return Status;
}