OvmfPkg/XenBusDxe: Add TestAndClearBit.

This atomically test's and clear's a bit.

Change in V3:
- adding IA32 support. (not yet reviewed)
  both XenBusDxe/Ia32/TestAndClearBit.{S,asm} are new

Change in V2:
- Adding .asm version
- Comment the function

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

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16266 6f19259b-4bc3-4df7-8a09-765794883524

OvmfPkg/XenBusDxe/XenBusDxe.h

@@ -97,4 +97,23 @@ struct _XENBUS_DEVICE {
   shared_info_t                 *SharedInfo;
 };
 
+/*
+ * Helpers
+ */
+
+/**
+  Atomically test and clear a bit.
+
+  @param Bit      Bit index to test in *Address
+  @param Address  The Address to the buffer that contain the bit to test.
+
+  @return Value of the Bit before it was cleared.
+**/
+INT32
+EFIAPI
+TestAndClearBit (
+  IN INT32 Bit,
+  IN volatile VOID *Address
+  );
+
 #endif