From 7dd17c26c0bb39b579cc2d5dd764ca6c79e23148 Mon Sep 17 00:00:00 2001
From: Martin Whitaker <pcmemtest@martin-whitaker.me.uk>
Date: Sat, 5 Feb 2022 17:58:17 +0000
Subject: [PATCH] Check whether the BIOS has enabled x2APIC mode, and disable
 SMP if so.

---
 system/cpuid.h |  4 +++-
 system/msr.h   |  3 ++-
 system/smp.c   | 15 +++++++++++++++
 3 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/system/cpuid.h b/system/cpuid.h
index 8bd8683..54acafc 100644
--- a/system/cpuid.h
+++ b/system/cpuid.h
@@ -84,7 +84,9 @@ typedef union {
         uint32_t    smx     : 1;
         uint32_t    eist    : 1;
         uint32_t    tm2     : 1;
-        uint32_t            : 23;   // ECX feature flags, bit 31
+        uint32_t            : 12;   // ECX feature flags, bit 20
+        uint32_t    x2apic  : 1;
+        uint32_t            : 10;   // ECX feature flags, bit 31
         uint32_t            : 29;   // EDX extended feature flags, bit 0
         uint32_t    lm      : 1;
         uint32_t            : 2;    // EDX extended feature flags, bit 31
diff --git a/system/msr.h b/system/msr.h
index 1e46c85..e044c74 100644
--- a/system/msr.h
+++ b/system/msr.h
@@ -4,7 +4,7 @@
 /*
  * Provides access to the CPU machine-specific registers.
  *
- * Copyright (C) 2020 Martin Whitaker.
+ * Copyright (C) 2020-2022 Martin Whitaker.
  */
 
 #define MSR_PLATFORM_INFO               0xce
@@ -12,6 +12,7 @@
 #define MSR_EBC_FREQUENCY_ID            0x2c
 
 #define MSR_IA32_PLATFORM_ID            0x17
+#define MSR_IA32_APIC_BASE              0x1b
 #define MSR_IA32_EBL_CR_POWERON         0x2a
 #define MSR_IA32_MCG_CTL                0x17b
 #define MSR_IA32_PERF_STATUS            0x198
diff --git a/system/smp.c b/system/smp.c
index 8151938..4a97dd5 100644
--- a/system/smp.c
+++ b/system/smp.c
@@ -21,6 +21,7 @@
 
 #include "cpuid.h"
 #include "memsize.h"
+#include "msr.h"
 #include "pmem.h"
 #include "string.h"
 #include "unistd.h"
@@ -58,6 +59,11 @@
 #define APIC_DELMODE_STARTUP        6
 #define APIC_DELMODE_EXTINT         7
 
+// IA32_APIC_BASE MSR bits
+
+#define IA32_APIC_ENABLED           (1 << 11)
+#define IA32_APIC_EXTENDED          (1 << 10)
+
 // Table signatures
 
 #define FPSignature     ('_' | ('M' << 8) | ('P' << 16) | ('_' << 24))
@@ -660,6 +666,15 @@ void smp_init(bool smp_enable)
 
     num_available_cpus = 1;
 
+    if (cpuid_info.flags.x2apic) {
+        uint32_t msrl, msrh;
+        rdmsr(MSR_IA32_APIC_BASE, msrl, msrh);
+        if ((msrl & IA32_APIC_ENABLED) && (msrl & IA32_APIC_EXTENDED)) {
+            // We don't currently support x2APIC mode.
+            smp_enable = false;
+        }
+    }
+
     if (smp_enable) {
         (void)(find_cpus_in_rsdp() || find_cpus_in_floating_mp_struct());