From 9e08e9f55f0403e8d940ba581a625ed181f6d117 Mon Sep 17 00:00:00 2001
From: "iliya.saroukha" <iliya.saroukhanian@etu.hesge.ch>
Date: Tue, 3 Dec 2024 15:40:09 +0100
Subject: [PATCH] feat: explained the relevant bits to VMX of the
IA32_FEATURE_CONTROL Model-Specific Register
---
docs/re.md | 27 +++++++++++++++++++++++++++
1 file changed, 27 insertions(+)
diff --git a/docs/re.md b/docs/re.md
index 349b862..1fae61c 100644
--- a/docs/re.md
+++ b/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/)
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