6 files changed, 81 insertions, 0 deletions
diff --git a/OvmfPkg/XenBusDxe/Ia32/TestAndClearBit.S b/OvmfPkg/XenBusDxe/Ia32/TestAndClearBit.S
new file mode 100644
index 0000000000..58dfa8ba57
--- /dev/null
+++ b/OvmfPkg/XenBusDxe/Ia32/TestAndClearBit.S
@@ -0,0 +1,13 @@
+# INT32
+# EFIAPI
+# TestAndClearBit (
+#   IN  INT32 Bit,
+#   IN  volatile VOID* Address
+#   );
+ASM_GLOBAL ASM_PFX(TestAndClearBit)
+ASM_PFX(TestAndClearBit):
+  mov 4(%esp), %ecx
+  mov 8(%esp), %edx
+  lock btrl %ecx, (%edx)
+  sbbl %eax, %eax
+  ret
diff --git a/OvmfPkg/XenBusDxe/Ia32/TestAndClearBit.asm b/OvmfPkg/XenBusDxe/Ia32/TestAndClearBit.asm
new file mode 100644
index 0000000000..ac809020a6
--- /dev/null
+++ b/OvmfPkg/XenBusDxe/Ia32/TestAndClearBit.asm
@@ -0,0 +1,17 @@
+.code
+
+; INT32
+; EFIAPI
+; TestAndClearBit (
+;   IN  INT32 Bit,
+;   IN  volatile VOID* Address
+;   );
+TestAndClearBit PROC
+  mov ecx, [esp + 4]
+  mov edx, [esp + 8]
+  lock btr [edx], ecx
+  sbb eax, eax
+  ret
+TestAndClearBit ENDP
+
+END
diff --git a/OvmfPkg/XenBusDxe/X64/TestAndClearBit.S b/OvmfPkg/XenBusDxe/X64/TestAndClearBit.S
new file mode 100644
index 0000000000..0372e83bdd
--- /dev/null
+++ b/OvmfPkg/XenBusDxe/X64/TestAndClearBit.S
@@ -0,0 +1,12 @@
+# INT32
+# EFIAPI
+# TestAndClearBit (
+#   IN  INT32 Bit,                // rcx
+#   IN  volatile VOID* Address    // rdx
+#   );
+ASM_GLOBAL ASM_PFX(TestAndClearBit)
+ASM_PFX(TestAndClearBit):
+  lock
+  btrl %ecx, (%rdx)
+  sbbl %eax, %eax
+  ret
diff --git a/OvmfPkg/XenBusDxe/X64/TestAndClearBit.asm b/OvmfPkg/XenBusDxe/X64/TestAndClearBit.asm
new file mode 100644
index 0000000000..3a2587913e
--- /dev/null
+++ b/OvmfPkg/XenBusDxe/X64/TestAndClearBit.asm
@@ -0,0 +1,16 @@
+.code
+
+; INT32
+; EFIAPI
+; TestAndClearBit (
+;   IN  INT32 Bit,                // rcx
+;   IN  volatile VOID* Address    // rdx
+;   );
+TestAndClearBit PROC
+  lock
+  btr [rdx], ecx
+  sbb eax, eax
+  ret
+TestAndClearBit ENDP
+
+END
diff --git a/OvmfPkg/XenBusDxe/XenBusDxe.h b/OvmfPkg/XenBusDxe/XenBusDxe.h
index 64579bb832..d80c118acf 100644
--- a/OvmfPkg/XenBusDxe/XenBusDxe.h
+++ b/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
diff --git a/OvmfPkg/XenBusDxe/XenBusDxe.inf b/OvmfPkg/XenBusDxe/XenBusDxe.inf
index 0af1946e26..742b7c615b 100644
--- a/OvmfPkg/XenBusDxe/XenBusDxe.inf
+++ b/OvmfPkg/XenBusDxe/XenBusDxe.inf
@@ -48,12 +48,16 @@
   Ia32/hypercall.asm
   Ia32/InterlockedCompareExchange16.S
   Ia32/InterlockedCompareExchange16.asm
+  Ia32/TestAndClearBit.S
+  Ia32/TestAndClearBit.asm
 
 [Sources.X64]
   X64/hypercall.S
   X64/hypercall.asm
   X64/InterlockedCompareExchange16.S
   X64/InterlockedCompareExchange16.asm
+  X64/TestAndClearBit.S
+  X64/TestAndClearBit.asm
 
 [LibraryClasses]
   UefiDriverEntryPoint