From ff94bd905431b9cdc3b3f376d029c769024579a0 Mon Sep 17 00:00:00 2001
From: Gerd Hoffmann <kraxel@redhat.com>
Date: Fri, 23 Aug 2024 11:55:31 +0200
Subject: [PATCH] OvmfPkg/PlatformInitLib: enable x2apic mode if needed

Enable x2apic mode in case the number of possible CPUs (including
hotplug-able CPus which are not (yet) online) is larger than 255.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit 8c8e05db24d8578cf87669e491f983fbd8357d55)
---
 OvmfPkg/Library/PlatformInitLib/Platform.c          | 6 ++++++
 OvmfPkg/Library/PlatformInitLib/PlatformInitLib.inf | 1 +
 2 files changed, 7 insertions(+)

diff --git a/OvmfPkg/Library/PlatformInitLib/Platform.c b/OvmfPkg/Library/PlatformInitLib/Platform.c
index 715533b1f2..afe8b0abd6 100644
--- a/OvmfPkg/Library/PlatformInitLib/Platform.c
+++ b/OvmfPkg/Library/PlatformInitLib/Platform.c
@@ -30,6 +30,7 @@
 #include <Library/QemuFwCfgS3Lib.h>
 #include <Library/QemuFwCfgSimpleParserLib.h>
 #include <Library/PciLib.h>
+#include <Library/LocalApicLib.h>
 #include <Guid/SystemNvDataGuid.h>
 #include <Guid/VariableFormat.h>
 #include <OvmfPlatforms.h>
@@ -720,6 +721,11 @@ PlatformMaxCpuCountInitialization (
     ));
   ASSERT (BootCpuCount <= MaxCpuCount);
 
+  if (MaxCpuCount > 255) {
+    DEBUG ((DEBUG_INFO, "%a: enable x2apic mode\n", __func__));
+    SetApicMode (LOCAL_APIC_MODE_X2APIC);
+  }
+
   PlatformInfoHob->PcdCpuMaxLogicalProcessorNumber  = MaxCpuCount;
   PlatformInfoHob->PcdCpuBootLogicalProcessorNumber = BootCpuCount;
 }
diff --git a/OvmfPkg/Library/PlatformInitLib/PlatformInitLib.inf b/OvmfPkg/Library/PlatformInitLib/PlatformInitLib.inf
index fb179e6791..c79b2ee106 100644
--- a/OvmfPkg/Library/PlatformInitLib/PlatformInitLib.inf
+++ b/OvmfPkg/Library/PlatformInitLib/PlatformInitLib.inf
@@ -48,6 +48,7 @@
   HobLib
   QemuFwCfgLib
   QemuFwCfgSimpleParserLib
+  LocalApicLib
   MemEncryptSevLib
   MemoryAllocationLib
   MtrrLib