eci: drivers/virtio: add vring_force_dma_api boot param

Prior to xdragon platform 20181230 release (e.g. 0930 release),
vring_use_dma_api() is required to return 'true' unconditionally.

Introduce a new kernel boot parameter called "vring_force_dma_api" to
control the behavior, boot xdragon host with "vring_force_dma_api"
command line to make ENI hotplug work, so that normal ECS hosts keep the
original behavior.
Reviewed-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Signed-off-by: NEryu Guan <eguan@linux.alibaba.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -4965,6 +4965,11 @@
 	vmpoff=		[KNL,S390] Perform z/VM CP command after power off.
 			Format: <command>
 
+	vring_force_dma_api
+			Force virtio vring to use dma api. This is only needed
+			on xdragon platform (prior to 20181230 release, e.g.
+			0930 release).
+
 	vsyscall=	[X86-64]
 			Controls the behavior of vsyscalls (i.e. calls to
 			fixed addresses of 0xffffffffff600x00 from legacy

drivers/virtio/virtio_ring.c

@@ -26,6 +26,8 @@
 #include <linux/dma-mapping.h>
 #include <xen/xen.h>
 
+static bool vring_force_dma_api = false;
+
 #ifdef DEBUG
 /* For development, we want to crash whenever the ring is screwed. */
 #define BAD_RING(_vq, fmt, args...)				\
@@ -115,6 +117,15 @@ struct vring_virtqueue {
 
 #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)
 
+static int __init vring_dma_api_setup(char *str)
+{
+	vring_force_dma_api = true;
+	printk(KERN_INFO "Force vring dma api enabled\n");
+
+	return 0;
+}
+__setup("vring_force_dma_api", vring_dma_api_setup);
+
 /*
  * Modern virtio devices have feature bits to specify whether they need a
  * quirk and bypass the IOMMU. If not there, just use the DMA API.
@@ -143,6 +154,13 @@ struct vring_virtqueue {
 
 static bool vring_use_dma_api(struct virtio_device *vdev)
 {
+	/*
+	 * Prior to xdragon platform 20181230 release (e.g. 0930 release), we
+	 * need this hack to get ENI hotplug to work.
+	 */
+	if (vring_force_dma_api)
+		return true;
+
 	if (!virtio_has_iommu_quirk(vdev))
 		return true;