[Xen-devel] [PATCHv3 1/4] x86: provide xadd()

[David Vrabel <david.vrabel@xxxxxxxxxx>]

xadd() atomically adds a value and returns the previous value.  This
is needed to implement ticket locks.

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
---
 xen/include/asm-x86/system.h |   47 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/xen/include/asm-x86/system.h b/xen/include/asm-x86/system.h
index 7111329..f244c8d 100644
--- a/xen/include/asm-x86/system.h
+++ b/xen/include/asm-x86/system.h
@@ -118,6 +118,53 @@ static always_inline unsigned long __cmpxchg(
 })
 
 /*
+ * Undefined symbol to cause link failure if a wrong size is used with
+ * xadd().
+ */
+extern unsigned long __bad_xadd_size(void);
+
+static always_inline unsigned long __xadd(
+    volatile void *ptr, unsigned long v, int size)
+{
+    switch ( size )
+    {
+    case 1:
+        asm volatile ( "lock; xaddb %b0,%1"
+                       : "+r" (v), "+m" (*__xg((volatile void *)ptr))
+                       :: "memory");
+        return v;
+    case 2:
+        asm volatile ( "lock; xaddw %w0,%1"
+                       : "+r" (v), "+m" (*__xg((volatile void *)ptr))
+                       :: "memory");
+        return v;
+    case 4:
+        asm volatile ( "lock; xaddl %k0,%1"
+                       : "+r" (v), "+m" (*__xg((volatile void *)ptr))
+                       :: "memory");
+        return v;
+    case 8:
+        asm volatile ( "lock; xaddq %q0,%1"
+                       : "+r" (v), "+m" (*__xg((volatile void *)ptr))
+                       :: "memory");
+
+        return v;
+    default:
+        return __bad_xadd_size();
+    }
+}
+
+/*
+ * Atomically add @v to the 1, 2, 4, or 8 byte value at @ptr.  Returns
+ * the previous value.
+ *
+ * This is a full memory barrier.
+ */
+#define xadd(ptr, v) ({                                         \
+            __xadd((ptr), (unsigned long)(v), sizeof(*(ptr)));  \
+        })
+
+/*
  * Both Intel and AMD agree that, from a programmer's viewpoint:
  *  Loads cannot be reordered relative to other loads.
  *  Stores cannot be reordered relative to other stores.
-- 
1.7.10.4


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