Add the VMM Communication Exception (#VC) to the InterruptDescriptorTable

For AMD SEV-ES and SEV-SNP, handling this exception is crucial for
functionality.

Signed-off-by: Harald Hoyer <[email protected]>
Co-authored-by: Philipp Oppermann <[email protected]>

Author: haraldh

src/structures/idt.rs

@@ -341,7 +341,36 @@ pub struct InterruptDescriptorTable {
     pub virtualization: Entry<HandlerFunc>,
 
     /// vector nr. 21-29
-    reserved_2: [Entry<HandlerFunc>; 9],
+    reserved_2: [Entry<HandlerFunc>; 8],
+
+    /// The VMM Communication Exception (`#VC`) is always generated by hardware when an `SEV-ES`
+    /// enabled guest is running and an `NAE` event occurs.
+    ///
+    /// `SEV-ES` stands for the _"Encrypted State"_ feature of the _"AMD Secure Encrypted Virtualization"_
+    /// technology. `NAE` stands for an _"Non-Automatic Exit"_, which is an `VMEXIT` event that requires
+    /// hypervisor emulation. See
+    /// [this whitepaper](https://www.amd.com/system/files/TechDocs/Protecting%20VM%20Register%20State%20with%20SEV-ES.pdf)
+    /// for an overview of the `SEV-ES` feature.
+    ///
+    /// The `#VC` exception is a precise, contributory, fault-type exception utilizing exception vector 29.
+    /// This exception cannot be masked. The error code of the `#VC` exception is equal
+    /// to the `#VMEXIT` code of the event that caused the `NAE`.
+    ///
+    /// In response to a `#VC` exception, a typical flow would involve the guest handler inspecting the error
+    /// code to determine the cause of the exception and deciding what register state must be copied to the
+    /// `GHCB` (_"Guest Hypervisor Communication Block"_) for the event to be handled. The handler
+    /// should then execute the `VMGEXIT` instruction to
+    /// create an `AE` and invoke the hypervisor. After a later `VMRUN`, guest execution will resume after the
+    /// `VMGEXIT` instruction where the handler can view the results from the hypervisor and copy state from
+    /// the `GHCB` back to its internal state as needed.
+    ///
+    /// Note that it is inadvisable for the hypervisor to set the `VMCB` (_"Virtual Machine Control Block"_)
+    /// intercept bit for the `#VC` exception as
+    /// this would prevent proper handling of `NAE`s by the guest. Similarly, the hypervisor should avoid
+    /// setting intercept bits for events that would occur in the `#VC` handler (such as `IRET`).
+    ///
+    /// The vector number of the ``#VC`` exception is 29.
+    pub vmm_communication_exception: Entry<HandlerFuncWithErrCode>,
 
     /// The Security Exception (`#SX`) signals security-sensitive events that occur while
     /// executing the VMM, in the form of an exception so that the VMM may take appropriate
@@ -407,7 +436,8 @@ impl InterruptDescriptorTable {
                 machine_check: Entry::missing(),
                 simd_floating_point: Entry::missing(),
                 virtualization: Entry::missing(),
-                reserved_2: [Entry::missing(); 9],
+                reserved_2: [Entry::missing(); 8],
+                vmm_communication_exception: Entry::missing(),
                 security_exception: Entry::missing(),
                 reserved_3: Entry::missing(),
                 interrupts: [Entry::missing(); 256 - 32],