OvmfPkg/VirtioLib: add function to map VRING

Add a function to map the ring buffer with BusMasterCommonBuffer so that
ring can be accessed by both guest and hypervisor.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
[lersek@redhat.com: fix typo in commit message]
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>

OvmfPkg/Include/Library/VirtioLib.h

@@ -60,6 +60,32 @@ VirtioRingInit (
   );
 
 
+/**
+
+  Map the ring buffer so that it can be accessed equally by both guest
+  and hypervisor.
+
+  @param[in]      VirtIo          The virtio device instance.
+
+  @param[in]      Ring            The virtio ring to map.
+
+  @param[out]     RingBaseShift   A resulting translation offset, to be
+                                  passed to VirtIo->SetQueueAddress().
+
+  @param[out]     Mapping         A resulting token to pass to
+                                  VirtIo->UnmapSharedBuffer().
+
+  @return         Status code from VirtIo->MapSharedBuffer()
+**/
+EFI_STATUS
+EFIAPI
+VirtioRingMap (
+  IN  VIRTIO_DEVICE_PROTOCOL *VirtIo,
+  IN  VRING                  *Ring,
+  OUT UINT64                 *RingBaseShift,
+  OUT VOID                   **Mapping
+  );
+
 /**
 
   Tear down the internal resources of a configured virtio ring.