Stefano Stabellini [off-list ref] wrote:

Could you elaborate why the existing hyper call won't work? 

+ * passed in with a new set of pages, contiguous and under 4G if so
+ * requested. The new pages are going to be "pinned": it's guaranteed
+ * that their p2m mapping won't be changed until explicitly
"unpinned".
+ * If return code is zero then @out.extent_list provides the MFNs of
the
+ * newly-allocated memory.  Returns zero on complete success,
otherwise
+ * a negative error code.
+ * On complete success then always @nr_exchanged == @in.nr_extents. 
On
+ * partial success @nr_exchanged indicates how much work was done.
+ */
+struct xen_get_dma_buf {
+    /*
+     * [IN] Details of memory extents to be exchanged (GMFN bases).
+     * Note that @in.address_bits is ignored and unused.
+     */
+    struct xen_memory_reservation in;
+
+    /*
+     * [IN/OUT] Details of new memory extents.
+     * We require that:
+     *  1. @in.domid == @out.domid
+     *  2. @in.nr_extents  << @in.extent_order == 
+     *     @out.nr_extents << @out.extent_order
+     *  3. @in.extent_start and @out.extent_start lists must not
overlap
+     *  4. @out.extent_start lists GPFN bases to be populated
+     *  5. @out.extent_start is overwritten with allocated GMFN bases
+     */
+    struct xen_memory_reservation out;
+
+    /*
+     * [OUT] Number of input extents that were successfully exchanged:
+     *  1. The first @nr_exchanged input extents were successfully
+     *     deallocated.
+     *  2. The corresponding first entries in the output extent list
correctly
+     *     indicate the GMFNs that were successfully exchanged.
+     *  3. All other input and output extents are untouched.
+     *  4. If not all input exents are exchanged then the return code
of this
+     *     command will be non-zero.
+     *  5. THIS FIELD MUST BE INITIALISED TO ZERO BY THE CALLER!
+     */
+    xen_ulong_t nr_exchanged;
+};
+DEFINE_GUEST_HANDLE_STRUCT(xen_get_dma_buf);
+
+#define XENMEM_put_dma_buf             27
+/*
+ * XENMEM_put_dma_buf unpins a set of pages, previously pinned by
+ * XENMEM_get_dma_buf. After this call the p2m mapping of the pages
can
+ * be transparently changed by the hypervisor, as usual. The pages are
+ * still accessible from the guest.
+ */
+struct xen_put_dma_buf {
+    /*
+     * [IN] Details of memory extents to be exchanged (GMFN bases).
+     * Note that @in.address_bits is ignored and unused.
+     */
+    struct xen_memory_reservation in;
+};
+DEFINE_GUEST_HANDLE_STRUCT(xen_put_dma_buf);
+
#endif /* __XEN_PUBLIC_MEMORY_H__ */