[PATCH v9 07/10] OvmfPkg/SmmCpuFeaturesLib: call CPU hot-eject handler

["Ankur Arora" <ankur.a.arora@oracle.com>]

Call the CPU hot-eject handler if one is installed. The condition for
installation is (PcdCpuMaxLogicalProcessorNumber > 1), and there's
a hot-unplug request.

The handler is called from SmmCpuFeaturesRendezvousExit(), which is
in-turn called at the tail-end of SmiRendezvous() after the BSP has
signalled an SMI exit via the "AllCpusInSync" loop.

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>
---

Notes:
    Addresses the following comments from v8:
    
    (1) Add a MemoryFence() before accessing mCpuHotEjctData->Handler
        (and comment to that effect.)

 .../Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.c  | 34 ++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.c b/OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.c
index 5c025bc717c3..fdf2380974fa 100644
--- a/OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.c
+++ b/OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.c
@@ -452,6 +452,40 @@ SmmCpuFeaturesRendezvousExit (
   IN UINTN  CpuIndex
   )
 {
+  //
+  // We only call the Handler if CPU hot-eject is enabled
+  // (PcdCpuMaxLogicalProcessorNumber > 1), and hot-eject is needed
+  // in this SMI exit (otherwise mCpuHotEjectData->Handler is not armed.)
+  //
+
+  if (mCpuHotEjectData != NULL) {
+    CPU_HOT_EJECT_HANDLER Handler;
+
+    //
+    // As the comment above mentions, mCpuHotEjectData->Handler might be
+    // written to on the BSP as part of handling of the CPU-ejection.
+    //
+    // We know that any initial assignment to mCpuHotEjectData->Handler
+    // (on the BSP, in the CpuHotplugMmi() context) is ordered-before the
+    // load below, since it is guaranteed to happen before the
+    // control-dependency of the BSP's SMI exit signal -- by way of a store
+    // to AllCpusInSync (on the BSP, in BspHandler()) and the corresponding
+    // AllCpusInSync loop (on the APs, in SmiRendezvous()) which depends on
+    // that store.
+    //
+    // This guarantees that these pieces of code can never execute
+    // simultaneously. In addition, we ensure that the following load is
+    // ordered-after the AllCpusInSync loop by using a MemoryFence() with
+    // acquire semantics.
+    //
+    MemoryFence();
+
+    Handler = mCpuHotEjectData->Handler;
+
+    if (Handler != NULL) {
+      Handler (CpuIndex);
+    }
+  }
 }
 
 /**
-- 
2.9.3