debug: invalidate ksKernelEntry on kernel exit

There's a few cases in the kernel where the ksKernelEntry tracking
is not perfect, such as in SError reporting, and (I believe) a few
other places which I haven't tracked down to a cause - but some of
e.g. the RISC-V trap code where the first entry faults and the 2nd
proceeds can report stale information.

In these cases, the kernel says that the entry was via a certain
syscall or interrupt (etc), even though that was clearly not the
case because we know the kernel exited. Now we will print out this:

    halting...
    Kernel entry via Unknown (0)

The changes:

- When exiting the kernel, via `c_exit_hook()`, reset
  `ksKernelEntry.path` to "Unknown".

  An alternative here would have been add a global "valid" boolean
  to the kernel state, but this requires modifying every site where
  we set the ksKernelEntry.path to also set valid = true, which is
  ugly.

- Remove Entry_UnimplementedDevice from entry_type_t as it is never
  used, to leave enough room to add Entry_Unknown.

- Switch out the CONFIG_DEBUG_BUILD || BENCHMARK TRACK ENTRIES #if
  in the x86 breakpoint code with the more concise
  `TRACK_KERNEL_ENTRIES` define used elsewhere.

Signed-off-by: julia <git.ts@trainwit.ch>

include/kernel/traps.h

@@ -41,6 +41,10 @@ static inline void c_exit_hook(void)
     }
 #endif /* CONFIG_BENCHMARK_TRACK_UTILISATION */
 
+#ifdef TRACK_KERNEL_ENTRIES
+    ksKernelEntry.path = Entry_Unknown;
+#endif
+
     arch_c_exit_hook();
 }
 

libsel4/include/sel4/benchmark_track_types.h

@@ -12,15 +12,19 @@
 #if (defined CONFIG_BENCHMARK_TRACK_KERNEL_ENTRIES || defined CONFIG_DEBUG_BUILD)
 
 /* the following code can be used at any point in the kernel
- * to determine detail about the kernel entry point */
+ * to determine detail about the kernel entry point.
+ * Note that all these values must fit in 3 bits for the `seL4_Word path: 3`
+ * bitfield below.
+ */
 typedef enum {
+    /* The kernel doesn't know why */
+    Entry_Unknown,
     Entry_Interrupt,
     Entry_UnknownSyscall,
     Entry_UserLevelFault,
     Entry_DebugFault,
     Entry_VMFault,
     Entry_Syscall,
-    Entry_UnimplementedDevice,
 #ifdef CONFIG_ARCH_ARM
     Entry_VCPUFault,
 #endif

src/arch/x86/machine/breakpoint.c

@@ -587,7 +587,7 @@ exception_t handleUserLevelDebugException(int int_vector)
     getAndResetActiveBreakpoint_t active_bp;
     testAndResetSingleStepException_t single_step_info;
 
-#if defined(CONFIG_DEBUG_BUILD) || defined(CONFIG_BENCHMARK_TRACK_KERNEL_ENTRIES)
+#ifdef TRACK_KERNEL_ENTRIES
     ksKernelEntry.path = Entry_UserLevelFault;
     ksKernelEntry.word = int_vector;
 #endif /* DEBUG */