XenGnttabBuffer: handle data offset for dmabufs

Handle additional data offset parameter while exporting and
importing dmabufs: dmabuf is backed by a scatter-gather table
and has offset parameter which tells where the actual data starts:
  - when dmabuf is created (exported) from grant references then
    data_ofs is used to set the offset field in the scatter list
    of the new dma-buf
  - when dma-buf is imported and then grant references provided
    to the user space then data_ofs is used to report that offset
    to user-space

Note! There is a dependency on libgnttab if it supports importing/
exporting dmabufs with data offset, so conditional compilation is
employed: new definition GNTTAB_HAS_DMABUF_OFFSET is set if the
library supports the offset. If it doesn't and a non-zero offset is
requested/provided, then a run-time exception is raised.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Oleksandr Grytsov <oleksandr_grytsov@epam.com>
Reviewed-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>

CMakeLists.txt

@@ -19,6 +19,24 @@ message(STATUS "XEN_INCLUDE_PATH              = ${XEN_INCLUDE_PATH}")
 message(STATUS "XEN_LIB_PATH                  = ${XEN_LIB_PATH}")
 message(STATUS)
 
+################################################################################
+# Autodetected options
+################################################################################
+
+include(CheckSymbolExists)
+
+# Test if version 2 of the xengnttab_dmabuf_imp_to_refs and
+# xengnttab_dmabuf_exp_from_refs functions are supported:
+# these functions have an additional data_ofs parameter
+# to set/get the offset of the data in the dma-buf's scatter-gather
+# table
+set(CMAKE_REQUIRED_LIBRARIES xengnttab)
+check_symbol_exists("xengnttab_dmabuf_imp_to_refs_v2" "xengnttab.h" GNTTAB_HAS_DMABUF_OFFSET)
+
+if(GNTTAB_HAS_DMABUF_OFFSET)
+	add_definitions(-DGNTTAB_HAS_DMABUF_OFFSET)
+endif()
+
 ################################################################################
 # Compiler flags
 ################################################################################

include/xen/be/XenGnttab.hpp

@@ -161,7 +161,8 @@ class XenGnttabDmaBufferExporter
 	 *            will be produced
 	 * @param[in] refs  vector of grant reference ids
 	 */
-	XenGnttabDmaBufferExporter(domid_t domId, const GrantRefs &refs);
+	XenGnttabDmaBufferExporter(domid_t domId, const GrantRefs &refs,
+							   size_t offset = 0);
 
 	int getFd() const { return mDmaBufFd; }
 
@@ -178,7 +179,7 @@ class XenGnttabDmaBufferExporter
 	xengnttab_handle* mHandle;
 	Log mLog;
 
-	void init(domid_t domId, const GrantRefs &refs);
+	void init(domid_t domId, const GrantRefs &refs, size_t offset);
 	void release();
 };
 
@@ -213,10 +214,16 @@ class XenGnttabDmaBufferImporter
 			delete;
 	~XenGnttabDmaBufferImporter();
 
+	/**
+	 * Returns offset of the data of the buffer.
+	 */
+	size_t offset() const { return mOffset; }
+
 private:
 	GrantRefs mRefs;
 	int mDmaBufFd;
 	xengnttab_handle* mHandle;
+	size_t mOffset;
 	Log mLog;
 
 	void init(domid_t domId, int fd, GrantRefs &refs);

src/XenGnttab.cpp

@@ -119,13 +119,14 @@ void XenGnttabBuffer::release()
  ******************************************************************************/
 
 XenGnttabDmaBufferExporter::XenGnttabDmaBufferExporter(domid_t domId,
-													   const GrantRefs &refs):
+													   const GrantRefs &refs,
+													   size_t offset) :
 	mDmaBufFd(-1),
 	mLog("XenGnttabDmaBufferExporter")
 {
 	try
 	{
-		init(domId, refs);
+		init(domId, refs, offset);
 	}
 	catch(const std::exception& e)
 	{
@@ -144,7 +145,8 @@ XenGnttabDmaBufferExporter::~XenGnttabDmaBufferExporter()
  * Private
  ******************************************************************************/
 
-void XenGnttabDmaBufferExporter::init(domid_t domId, const GrantRefs &refs)
+void XenGnttabDmaBufferExporter::init(domid_t domId, const GrantRefs &refs,
+									  size_t offset)
 {
 	uint32_t fd;
 	int ret;
@@ -155,8 +157,21 @@ void XenGnttabDmaBufferExporter::init(domid_t domId, const GrantRefs &refs)
 					  << domId << ", count: " << refs.size();
 
 	/* Always allocate the buffer to be DMA capable. */
+#ifdef GNTTAB_HAS_DMABUF_OFFSET
+	DLOG(mLog, DEBUG) << "DMA buffer offset: " << offset;
+
+	ret = xengnttab_dmabuf_exp_from_refs_v2(mHandle, domId, GNTDEV_DMA_FLAG_WC,
+											refs.size(), refs.data(), &fd,
+											offset);
+#else
+	if (offset)
+	{
+		throw XenGnttabException("Can't produce DMA buffer from grant references with non-zero offset",
+								 ENOTSUP);
+	}
 	ret = xengnttab_dmabuf_exp_from_refs(mHandle, domId, GNTDEV_DMA_FLAG_WC,
 										 refs.size(), refs.data(), &fd);
+#endif
 
 	if (ret)
 	{
@@ -228,15 +243,23 @@ XenGnttabDmaBufferImporter::~XenGnttabDmaBufferImporter()
 
 void XenGnttabDmaBufferImporter::init(domid_t domId, int fd, GrantRefs &refs)
 {
+	uint32_t offset;
 	int ret;
 
 	mHandle = XenGnttab::getHandle();
 
 	DLOG(mLog, DEBUG) << "Produce grant references from DMA buffer, dom: "
 					  << domId << ", fd: " << fd << ", count: " << refs.size();
 
+#ifdef GNTTAB_HAS_DMABUF_OFFSET
+	ret = xengnttab_dmabuf_imp_to_refs_v2(mHandle, domId, fd,
+										  refs.size(), refs.data(),
+										  &offset);
+#else
 	ret = xengnttab_dmabuf_imp_to_refs(mHandle, domId, fd,
 									   refs.size(), refs.data());
+	offset = 0;
+#endif
 
 	if (ret)
 	{
@@ -245,6 +268,7 @@ void XenGnttabDmaBufferImporter::init(domid_t domId, int fd, GrantRefs &refs)
 	}
 
 	mDmaBufFd = fd;
+	mOffset = static_cast<size_t>(offset);
 }
 
 void XenGnttabDmaBufferImporter::release()