feat: explained the relevant bits to VMX of the IA32_FEATURE_CONTROL Model-Specific Register

docs/re.md

@@ -109,6 +109,33 @@ static void enable_vmx(void) {
 }
 ```
 
+VMXON is also controlled by the IA32_FEATURE_CONTROL MSR (Model-Specific
+Register). MSRs are 64-bit wide. The address of IA32_FEATURE_CONTROL MSR is
+**0x3a**.
+
+The relevant bits (in terms of VMX) of this MSR are:
+
+- Bit 0 (Lock bit)
+    - System BIOS can use this bit to provide a setup option for BIOS to disable
+    support for VMX. **If this bit is set, WRMSR to this MSR will cause a general-
+    protection exception**. To enable VMX support in a platform, BIOS must set bit
+    1, bit 2, or both (see below), as well as the lock bit.
+- Bit 1 (VMXON in SMX operation)
+- Bit 2 (VMXON outside SMX operation)
+
+**In my case, after having activated VT-x through the BIOS, the value returned
+by the IA32_FEATURE_CONTROL MSR is always equal to 5 (e.g. the lock bit and
+VMXON outside SMX operation are always set).**
+
+**If I decide to deactivate VT-x, the MSR will return the value of 1 (e.g. lock
+bit).**
+
+Due to the fact that the **lock bit** is always **set**, it means that I'm not
+able to write to this MSR using __wrmsr. What's interesting about this is that
+the tutorial tries to modify this MSR but if we assume that the **lock bit is set**
+that branch **will be a no-op**.
+
+
 ## References
 
 [^1]: [The Linux Kernel Module Programming Guide](https://sysprog21.github.io/lkmpg/)