From 579b338ce43a0f084d22b082c82e341f81ed7e2f Mon Sep 17 00:00:00 2001
From: Ian Moffett <ian@osmora.org>
Date: Tue, 23 Dec 2025 01:44:33 -0500
Subject: mos/x86_64+mu: Add processor control region

Signed-off-by: Ian Moffett <ian@osmora.org>
---
 mos/sys/arch/x86_64/cpu/cpu.c | 22 ++++++++++++++++++++++
 mos/sys/inc/mu/cpu.h          | 17 +++++++++++++++++
 mos/sys/kern/kern_init.c      |  6 ++++++
 3 files changed, 45 insertions(+)
 create mode 100644 mos/sys/arch/x86_64/cpu/cpu.c

diff --git a/mos/sys/arch/x86_64/cpu/cpu.c b/mos/sys/arch/x86_64/cpu/cpu.c
new file mode 100644
index 0000000..e5f7479
--- /dev/null
+++ b/mos/sys/arch/x86_64/cpu/cpu.c
@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) 2025, Ian Moffett.
+ * Provided under the BSD-3 clause.
+ */
+
+#include <sdk/types.h>
+#include <kern/trace.h>
+#include <md/msr.h>
+#include <mu/cpu.h>
+
+#define dtrace(fmt, ...) trace("cpu: " fmt, ##__VA_ARGS__)
+
+void
+mu_cpu_conf(PCR *pcr)
+{
+    if (pcr == NULL) {
+        return;
+    }
+
+    md_wrmsr(IA32_GS_BASE, (UPTR)pcr);
+    dtrace("pcr engaged\n");
+}
diff --git a/mos/sys/inc/mu/cpu.h b/mos/sys/inc/mu/cpu.h
index 9856df8..2fb4e6e 100644
--- a/mos/sys/inc/mu/cpu.h
+++ b/mos/sys/inc/mu/cpu.h
@@ -8,6 +8,16 @@
 
 #include <sdk/types.h>
 
+/*
+ * The processor control region represents the currently
+ * running processor
+ *
+ * @logical_id: Assigned by us
+ */
+typedef struct {
+    UBYTE logical_id;
+} PCR;
+
 /*
  * Get the current IRQ state
  *
@@ -33,6 +43,13 @@ void mu_irq_setmask(BOOL mask);
  */
 USIZE mu_cpu_aswap(USIZE *p, USIZE val);
 
+/*
+ * Configure the current processor
+ *
+ * @pcr: Processor control region
+ */
+void mu_cpu_conf(PCR *pcr);
+
 /*
  * Halt the current processor core
  */
diff --git a/mos/sys/kern/kern_init.c b/mos/sys/kern/kern_init.c
index 2502dc2..c8f9283 100644
--- a/mos/sys/kern/kern_init.c
+++ b/mos/sys/kern/kern_init.c
@@ -6,6 +6,7 @@
 #include <kern/trace.h>
 #include <kern/bpt.h>
 #include <mu/uart.h>
+#include <mu/cpu.h>
 #include <mm/pmm.h>
 
 #define write_boot_header()                     \
@@ -19,6 +20,8 @@
 /* Forward declaration */
 void kern_main(void);
 
+static PCR bsp_pcr;
+
 void
 kern_main(void)
 {
@@ -33,4 +36,7 @@ kern_main(void)
 
     /* Initialize the physical memory manager */
     pmm_init();
+
+    /* Initialize the BSP PCR */
+    mu_cpu_conf(&bsp_pcr);
 }
-- 
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