Ovmf/Xen: move XenBusDxe hypercall code to separate library

This moves all of the Xen hypercall code that was private to XenBusDxe
to a new library class XenHypercallLib. This will allow us to reimplement
it for ARM, and to export the Xen hypercall functionality to other parts
of the code, such as a Xen console SerialPortLib driver.

Contributed-under: TianoCore Contribution Agreement 1.0
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16970 6f19259b-4bc3-4df7-8a09-765794883524

OvmfPkg/Library/XenHypercallLib/X64/hypercall.nasm

@@ -0,0 +1,26 @@
+DEFAULT REL
+SECTION .text
+
+; INTN
+; EFIAPI
+; __XenHypercall2 (
+;   IN     VOID *HypercallAddr,
+;   IN OUT INTN Arg1,
+;   IN OUT INTN Arg2
+;   );
+global ASM_PFX(__XenHypercall2)
+ASM_PFX(__XenHypercall2):
+  push rdi
+  push rsi
+  ; Copy HypercallAddr to rax
+  mov rax, rcx
+  ; Copy Arg1 to the register expected by Xen
+  mov rdi, rdx
+  ; Copy Arg2 to the register expected by Xen
+  mov rsi, r8
+  ; Call HypercallAddr
+  call rax
+  pop rsi
+  pop rdi
+  ret
+