OvmfPkg/XenBusDxe: Add InterlockedCompareExchange16.

This patch is inspired by InterlockedCompareExchange32 from the
BaseSynchronizationLib.

The function will be used in the "OvmfPkg/XenBusDxe: Add Grant Table
functions" patch.

Change in V3:
- Implement both .S and .asm, to get rid of GCC specific asm.
- Implement 32bit part of the assembly

Change in V2:
- Add intel compilation code
  MSFT code is not compied over because I don't know how it works.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16263 6f19259b-4bc3-4df7-8a09-765794883524

OvmfPkg/XenBusDxe/InterlockedCompareExchange16.c

@@ -0,0 +1,33 @@
+#include <Library/DebugLib.h>
+#include "InterlockedCompareExchange16.h"
+
+/**
+  Performs an atomic compare exchange operation on a 16-bit unsigned integer.
+
+  Performs an atomic compare exchange operation on the 16-bit unsigned integer
+  specified by Value.  If Value is equal to CompareValue, then Value is set to
+  ExchangeValue and CompareValue is returned.  If Value is not equal to CompareValue,
+  then Value is returned.  The compare exchange operation must be performed using
+  MP safe mechanisms.
+
+  If Value is NULL, then ASSERT().
+
+  @param  Value         A pointer to the 16-bit value for the compare exchange
+                        operation.
+  @param  CompareValue  16-bit value used in compare operation.
+  @param  ExchangeValue 16-bit value used in exchange operation.
+
+  @return The original *Value before exchange.
+
+**/
+UINT16
+EFIAPI
+InterlockedCompareExchange16 (
+  IN OUT  UINT16                    *Value,
+  IN      UINT16                    CompareValue,
+  IN      UINT16                    ExchangeValue
+  )
+{
+  ASSERT (Value != NULL);
+  return InternalSyncCompareExchange16 (Value, CompareValue, ExchangeValue);
+}