From 87a4bc91d84ee7eca4ca25ef005542cfbc4bbea6 Mon Sep 17 00:00:00 2001
From: apio <blobs.trading@gmail.com>
Date: Fri, 6 Jan 2023 17:31:22 +0100
Subject: [PATCH] arch/x86_64: Nicer display for page faults that shows a
 human-readable version of the error code

---
 kernel/src/arch/x86_64/CPU.cpp | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/kernel/src/arch/x86_64/CPU.cpp b/kernel/src/arch/x86_64/CPU.cpp
index 72289888..3f88e237 100644
--- a/kernel/src/arch/x86_64/CPU.cpp
+++ b/kernel/src/arch/x86_64/CPU.cpp
@@ -29,6 +29,19 @@ extern void setup_idt();
     kerrorln("FIXME(interrupt): %s", name);                                                                            \
     CPU::efficient_halt();
 
+#define PF_PRESENT 1 << 0
+#define PF_WRITE 1 << 1
+#define PF_USER 1 << 2
+#define PF_RESERVED 1 << 3
+#define PF_NX_VIOLATION 1 << 4
+
+void decode_page_fault_error_code(u64 code)
+{
+    kwarnln("Fault details: %s | %s | %s%s%s", (code & PF_PRESENT) ? "Present" : "Not present",
+            (code & PF_WRITE) ? "Write access" : "Read access", (code & PF_USER) ? "User mode" : "Kernel mode",
+            (code & PF_RESERVED) ? " | Reserved bits set" : "", (code & PF_NX_VIOLATION) ? " | NX violation" : "");
+}
+
 [[noreturn]] void handle_page_fault(Registers* regs)
 {
     CPU::disable_interrupts();
@@ -37,6 +50,8 @@ extern void setup_idt();
     asm volatile("mov %%cr2, %0" : "=r"(cr2));
     kerrorln("Page fault at RIP %lx while accessing %lx!", regs->rip, cr2);
 
+    decode_page_fault_error_code(regs->error);
+
     CPU::print_stack_trace_at(regs);
 
     CPU::efficient_halt();