[Xen-devel] [PATCHv2 4/6] 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 |   29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/xen/include/asm-x86/system.h b/xen/include/asm-x86/system.h
index 7111329..1e6c6a8 100644
--- a/xen/include/asm-x86/system.h
+++ b/xen/include/asm-x86/system.h
@@ -117,6 +117,35 @@ static always_inline unsigned long __cmpxchg(
                                    (unsigned long)__n,sizeof(*(ptr)))); \
 })
 
+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)));
+        return v;
+    case 2:
+        asm volatile ( "lock; xaddw %w0,%1"
+                       : "+r" (v), "+m" (*__xg((volatile void *)ptr)));
+        return v;
+    case 4:
+        asm volatile ( "lock; xaddl %k0,%1"
+                       : "+r" (v), "+m" (*__xg((volatile void *)ptr)));
+        return v;
+    case 8:
+        asm volatile ( "lock; xaddq %q0,%1"
+                       : "+r" (v), "+m" (*__xg((volatile void *)ptr)));
+        return v;
+    }
+    return 0;
+}
+
+#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.
-- 
1.7.10.4


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