Re: [PATCH] x86/cmpxchg: Add safety for bad sizes

[Nicola Vetrini]

On 2026-01-27 16:53, Jan Beulich wrote:

On 27.01.2026 11:23, Andrew Cooper wrote:

@@ -66,6 +68,8 @@ static always_inline unsigned long __xchg(

: [x] "+r" (x), [ptr] "+m" (*(volatile uint64_t *)ptr)

                        :: "memory" );
         break;
+    default:
+        __bad_xchg_size();

What has come of the plans to emit an assembly error directive in such
situations?

Also for Misra's sake "break" will be wanted.

Or mark the function noreturn for instance

@@ -106,6 +110,8 @@ static always_inline unsigned long __cmpxchg(
                        : [new] "r" (new), "a" (old)
                        : "memory" );
         return prev;
+    default:
+        BUG();
     }
     return old;
 }
@@ -137,6 +143,8 @@ static always_inline unsigned long cmpxchg_local_(
                        : "=a" (prev), [ptr] "+m" (*(uint64_t *)ptr)
                        : [new] "r" (new), "a" (old) );
         break;
+    default:
+        BUG();
     }

     return prev;

Hmm. If for some reason hvmemul_cmpxchg() ended up hitting either of these,

we'd immediately have an XSA. Imo these want to be ASSERT_UNREACHABLE()
with plausible recovery for release builds.

Jan

--
Nicola Vetrini, B.Sc.
Software Engineer
BUGSENG (https://bugseng.com)
LinkedIn: https://www.linkedin.com/in/nicola-vetrini-a42471253