Designate a worker CPU (we use the one executing the root MMI handler),
which will do the actual ejection via QEMU in CpuEject().

CpuEject(), on the worker CPU, ejects each marked CPU by first
selecting its APIC ID and then sending the QEMU "eject" command.
QEMU in-turn signals the remote VCPU thread which context-switches
it out of the SMI.

CpuEject(), on the CPU being ejected, spins around in its holding
area until this final context-switch. This does mean that there is
some CPU state that would ordinarily be restored (in SmiRendezvous()
and in SmiEntry.nasm::CommonHandler), but will not be anymore.
This unrestored state includes FPU state, CET enable, stuffing of
RSB and the final RSM. Since the CPU state is destroyed by QEMU,
this should be okay.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Aaron Young <aaron.young@oracle.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3132
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 OvmfPkg/CpuHotplugSmm/CpuHotplug.c | 66 +++++++++++++++++++++++++++++++++++---
 1 file changed, 61 insertions(+), 5 deletions(-)

diff --git a/OvmfPkg/CpuHotplugSmm/CpuHotplug.c b/OvmfPkg/CpuHotplugSmm/CpuHotplug.c
index e8ba9ae59e69..27fd982d6771 100644
--- a/OvmfPkg/CpuHotplugSmm/CpuHotplug.c
+++ b/OvmfPkg/CpuHotplugSmm/CpuHotplug.c
@@ -200,6 +200,54 @@ CpuEject(
     return;
   }
 
+  if (ApicId == CPU_EJECT_WORKER) {
+    UINT32 CpuIndex;
+
+    for (CpuIndex = 0; CpuIndex < mCpuHotEjectData->ArrayLength; CpuIndex++) {
+      UINT64 RemoveApicId = mCpuHotEjectData->ApicIdMap[CpuIndex];
+
+      if ((RemoveApicId != CPU_EJECT_INVALID &&
+           RemoveApicId != CPU_EJECT_WORKER)) {
+        //
+        // This to-be-ejected-CPU has already received the BSP's SMI exit
+        // signal and, will execute SmmCpuFeaturesSmiRendezvousExit()
+        // followed by this callback or is already waiting in the
+        // CpuDeadLoop() below.
+        //
+        // Tell QEMU to context-switch it out.
+        //
+        QemuCpuhpWriteCpuSelector (mMmCpuIo, RemoveApicId);
+        QemuCpuhpWriteCpuStatus (mMmCpuIo, QEMU_CPUHP_STAT_EJECTED);
+
+        //
+        // Compiler barrier to ensure the next store isn't reordered
+        //
+        MemoryFence();
+
+        //
+        // Clear the eject status for CpuIndex to ensure that an invalid
+        // SMI later does not end up trying to eject it or a newly
+        // hotplugged CpuIndex does not go into the dead loop.
+        //
+        mCpuHotEjectData->ApicIdMap[CpuIndex] = CPU_EJECT_INVALID;
+
+        DEBUG ((DEBUG_INFO, "%a: Unplugged CPU %u -> " FMT_APIC_ID "\n",
+               __FUNCTION__, CpuIndex, RemoveApicId));
+      }
+    }
+
+    //
+    // Clear our own worker status.
+    //
+    mCpuHotEjectData->ApicIdMap[ProcessorNum] = CPU_EJECT_INVALID;
+
+    //
+    // We are done until the next hot-unplug; clear the handler.
+    //
+    mCpuHotEjectData->Handler = NULL;
+    return;
+  }
+
   //
   // CPU(s) being unplugged get here from SmmCpuFeaturesSmiRendezvousExit()
   // after having been cleared to exit the SMI by the monarch and thus have
@@ -296,6 +344,19 @@ UnplugCpus(
   }
 
   if (EjectCount) {
+    UINTN  Worker;
+
+    Status = mMmCpuService->WhoAmI(mMmCpuService, &Worker);
+    ASSERT_EFI_ERROR(Status);
+    //
+    // UnplugCpus() is called via the root MMI handler and thus we are
+    // executing in the BSP context.
+    //
+    // Mark ourselves as the worker CPU.
+    //
+    ASSERT (mCpuHotEjectData->ApicIdMap[Worker] == CPU_EJECT_INVALID);
+    mCpuHotEjectData->ApicIdMap[Worker] = CPU_EJECT_WORKER;
+
     //
     // We have processors to be ejected; install the handler.
     //
@@ -419,11 +480,6 @@ CpuHotplugMmi (
   if (EFI_ERROR (Status)) {
     goto Fatal;
   }
-  if (ToUnplugCount > 0) {
-    DEBUG ((DEBUG_ERROR, "%a: hot-unplug is not supported yet\n",
-      __FUNCTION__));
-    goto Fatal;
-  }
 
   if (PluggedCount > 0) {
     Status = PlugCpus(mPluggedApicIds, PluggedCount);
-- 
2.9.3



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