From 1e1c389caf93e089f1cb36a5b05dce679585fb98 Mon Sep 17 00:00:00 2001
From: Josh Holtrop <jholtrop@gmail.com>
Date: Thu, 27 Oct 2022 23:42:18 -0400
Subject: [PATCH] Turn on EFER NXE bit to allow no-execute pages

---
 src/hulk/cpu.d  | 18 ++++++++++++++++++
 src/hulk/hurl.d |  2 ++
 2 files changed, 20 insertions(+)

diff --git a/src/hulk/cpu.d b/src/hulk/cpu.d
index fa4d27c..00906d1 100644
--- a/src/hulk/cpu.d
+++ b/src/hulk/cpu.d
@@ -5,7 +5,20 @@ module hulk.cpu;
 
 import ldc.llvmasm;
 
+/** MSR register numbers. @{ */
 enum uint MSR_EFER = 0xC000_0080u;
+/** @} */
+
+/** EFER flags. @{ */
+enum uint EFER_SCE = 0x1u;
+enum uint EFER_LME = 0x100u;
+enum uint EFER_LMA = 0x400u;
+enum uint EFER_NXE = 0x800u;
+enum uint EFER_SVME = 0x1000u;
+enum uint EFER_LMSLE = 0x2000u;
+enum uint EFER_FFXSR = 0x4000u;
+enum uint EFER_TCE = 0x8000u;
+/** @} */
 
 void cli()
 {
@@ -55,6 +68,11 @@ ulong rdmsr(uint msr)
             or %rdx, %rax`, "={rax},{ecx},~{rdx}", msr);
 }
 
+void wrmsr(uint msr, ulong value)
+{
+    __asm(`wrmsr`, "{ecx},{edx},{eax}", msr, value >> 32u, value);
+}
+
 ubyte in8(ushort ioaddr)
 {
     return __asm!ubyte("inb %dx, %al", "={al},{dx}", ioaddr);
diff --git a/src/hulk/hurl.d b/src/hulk/hurl.d
index 621ca08..7509280 100644
--- a/src/hulk/hurl.d
+++ b/src/hulk/hurl.d
@@ -33,6 +33,8 @@ struct hurl
      */
     public static void initialize(HulkHeader * header)
     {
+        /* Turn on NXE (no execute enable) flag in the EFER MSR. */
+        wrmsr(MSR_EFER, rdmsr(MSR_EFER) | EFER_NXE);
         m_pt_base = allocate_pt();
         size_t hulk_bin_phys_size = cast(ulong)header.total_size - LinkerAddresses.hulk_bss_size;
         /* Identity map all physical RAM. */