x86/kvm: Make VirtIO use DMA API in KVM guest

VirtIO for KVM is a primary way to provide IO. All memory that used for
communication with the host has to be marked as shared.

The easiest way to archive that is to use DMA API that already knows how
to deal with shared memory.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

drivers/virtio/virtio_ring.c

@@ -12,6 +12,7 @@
 #include <linux/hrtimer.h>
 #include <linux/dma-mapping.h>
 #include <xen/xen.h>
+#include <asm/kvm_para.h>
 
 #ifdef DEBUG
 /* For development, we want to crash whenever the ring is screwed. */
@@ -255,6 +256,9 @@ static bool vring_use_dma_api(struct virtio_device *vdev)
 	if (xen_domain())
 		return true;
 
+	if (kvm_mem_protected())
+		return true;
+
 	return false;
 }