AR: Clarify mcontext/hcontext.

It was a bit confusing before. Hopefully now it's more clear how they
relate, and why you might want each one.

xml/hwbp_registers.xml

@@ -218,15 +218,13 @@
     </register>
 
     <register name="Hypervisor Context" short="hcontext" address="0x6a8">
-        This optional register is only accessible in S/HS-mode, M-mode and
-        Debug Mode.
+        This optional register may only be implemented if the H extension is
+        implemented. If it is implemented, \RcsrMcontext must also be implemented.
 
-        Accessibility of this CSR is controlled by \Rmstateenzero[57]
-        in the Smstateen extension.
+        This register is only accessible in S/HS-Mode, M-mode and Debug Mode.
 
-        If the H extension is not implemented then this register is not
-        implemented, though the underlying state may be accessible via
-        the optional \RcsrMcontext alias.
+        if Smstateen is implemented, then accessibility of this CSR in S/HS-Mode
+        is controlled by \Rmstateenzero[57].
 
         <field name="hcontext" bits="XLEN-1:0" access="WARL" reset="0">
             Hypervisor mode software can write a context number to this register,
@@ -262,8 +260,22 @@
     </register>
 
     <register name="Machine Context" short="mcontext" address="0x7a8">
-        This optional register is an alias for \RcsrHcontext and is only
-        accessible in M-mode and Debug mode.
+        This register must be implemented if \RcsrHcontext is implemented, and
+        is optional otherwise.  It is only accessible in M-mode and Debug mode.
+
+        This register provides access to the \FcsrHcontextHcontext field, even
+        if the \RcsrHcontext register is not implemented.
+
+        \begin{commentary}
+            \FcsrHcontextHcontext is primarily useful to set triggers on
+            hypervisor systems that only fire when a given VM is executing. It
+            is also useful in systems where M-Mode implements something like a
+            hypervisor directly.
+        \end{commentary}
+
+        <field name="mcontext" bits="XLEN-1:0" access="WARL" reset="0">
+            This field is an alias for \FcsrHcontextHcontext.
+        </field>
     </register>
 
     <register name="Machine Supervisor Context" short="mscontext" address="0x7aa">