[PATCH] misra: add ASSERT_UNREACHABLE() in default clauses

[Dmytro Prokopchuk1 <dmytro_prokopchuk1@xxxxxxxx>]

MISRA Rule 16.4: Every switch statement shall have a default label.
The default clause must contain either a statement or a comment
prior to its terminating break statement.

However, there is a documented rule that apply to the Xen in
'docs/misra/rules.rst':
Switch statements with integer types as controlling expression
should have a default label:
 - if the switch is expected to handle all possible cases
  explicitly, then a default label shall be added to handle
  unexpected error conditions, using BUG(), ASSERT(), WARN(),
  domain_crash(), or other appropriate methods;

These changes add `ASSERT_UNREACHABLE()` macro to the default clause of
switch statements that already explicitly handle all possible cases. This
ensures compliance with MISRA, avoids undefined behavior in unreachable
paths, and helps detect errors during development.

Signed-off-by: Dmytro Prokopchuk <dmytro_prokopchuk1@xxxxxxxx>
---
 xen/arch/arm/decode.c      |  3 +++
 xen/arch/arm/guest_walk.c  |  4 ++++
 xen/common/grant_table.c   | 10 ++++++++--
 xen/drivers/char/console.c |  3 +++
 4 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/xen/arch/arm/decode.c b/xen/arch/arm/decode.c
index 2537dbebc1..cb64137b3b 100644
--- a/xen/arch/arm/decode.c
+++ b/xen/arch/arm/decode.c
@@ -178,6 +178,9 @@ static int decode_thumb(register_t pc, struct hsr_dabt 
*dabt)
         case 3: /* Signed byte */
             update_dabt(dabt, reg, 0, true);
             break;
+        default:
+            ASSERT_UNREACHABLE();
+            break;
         }
 
         break;
diff --git a/xen/arch/arm/guest_walk.c b/xen/arch/arm/guest_walk.c
index 09fe486598..9199a29602 100644
--- a/xen/arch/arm/guest_walk.c
+++ b/xen/arch/arm/guest_walk.c
@@ -167,6 +167,10 @@ static bool guest_walk_sd(const struct vcpu *v,
             *perms |= GV2M_EXEC;
 
         break;
+
+        default:
+            ASSERT_UNREACHABLE();
+            break;
     }
 
     return true;
diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index cf131c43a1..60fc47f0c8 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -330,9 +330,12 @@ shared_entry_header(struct grant_table *t, grant_ref_t ref)
         /* Returned values should be independent of speculative execution */
         block_speculation();
         return &shared_entry_v2(t, ref).hdr;
+
+    default:
+        ASSERT_UNREACHABLE();
+        break;
     }
 
-    ASSERT_UNREACHABLE();
     block_speculation();
 
     return NULL;
@@ -727,10 +730,13 @@ static unsigned int nr_grant_entries(struct grant_table 
*gt)
         /* Make sure we return a value independently of speculative execution 
*/
         block_speculation();
         return f2e(nr_grant_frames(gt), 2);
+
+    default:
+        ASSERT_UNREACHABLE();
+        break;
 #undef f2e
     }
 
-    ASSERT_UNREACHABLE();
     block_speculation();
 
     return 0;
diff --git a/xen/drivers/char/console.c b/xen/drivers/char/console.c
index 9bd5b4825d..608616f2af 100644
--- a/xen/drivers/char/console.c
+++ b/xen/drivers/char/console.c
@@ -889,6 +889,9 @@ static int cf_check parse_console_timestamps(const char *s)
         opt_con_timestamp_mode = TSM_DATE;
         con_timestamp_mode_upd(param_2_parfs(parse_console_timestamps));
         return 0;
+    default:
+        ASSERT_UNREACHABLE();
+        break;
     }
     if ( *s == '\0' || /* Compat for old booleanparam() */
          !strcmp(s, "date") )
-- 
2.43.0