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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

All other architectures helpers have safey against passing a bad size, but the
x86 forms did not.  For __xchg(), use DCE against a function which doesn't
exist.

For hvmemul_cmpxchg() this doesn't work as the size is a parameter rather than
known at compile time.  Use BUG() to detect runtime malfunctions.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

bloat-o-meter reports:

  Function                                     old     new   delta
  hvmemul_cmpxchg                             1116    1092     -24

which is surely down to the hidden __builtin_unreachable() causing some
codepaths to be omitted.
---
 xen/arch/x86/include/asm/system.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/xen/arch/x86/include/asm/system.h 
b/xen/arch/x86/include/asm/system.h
index 6c2800d8158d..1074786a9d23 100644
--- a/xen/arch/x86/include/asm/system.h
+++ b/xen/arch/x86/include/asm/system.h
@@ -36,6 +36,8 @@ static inline void clwb(const void *p)
 
 #include <asm/x86_64/system.h>
 
+extern unsigned long __bad_xchg_size(void);
+
 /*
  * Note: no "lock" prefix even on SMP: xchg always implies lock anyway
  * Note 2: xchg has side effect, so that attribute volatile is necessary,
@@ -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();
     }
     return x;
 }
@@ -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;
-- 
2.39.5