Re: [RFC PATCH] virtio_net: XDP support for adjust_head

On 17-01-02 11:44 AM, John Fastabend wrote:

Add support for XDP adjust head by allocating a 256B header region
that XDP programs can grow into. This is only enabled when a XDP
program is loaded.

In order to ensure that we do not have to unwind queue headroom push
queue setup below bpf_prog_add. It reads better to do a prog ref
unwind vs another queue setup call.

: There is a problem with this patch as is. When xdp prog is loaded
  the old buffers without the 256B headers need to be flushed so that
  the bpf prog has the necessary headroom. This patch does this by
  calling the virtqueue_detach_unused_buf() and followed by the
  virtnet_set_queues() call to reinitialize the buffers. However I
  don't believe this is safe per comment in virtio_ring this API
  is not valid on an active queue and the only thing we have done
  here is napi_disable/napi_enable wrappers which doesn't do anything
  to the emulation layer.

  So the RFC is really to find the best solution to this problem.
  A couple things come to mind, (a) always allocate the necessary
  headroom but this is a bit of a waste (b) add some bit somewhere
  to check if the buffer has headroom but this would mean XDP programs
  would be broke for a cycle through the ring, (c) figure out how
  to deactivate a queue, free the buffers and finally reallocate.
  I think (c) is the best choice for now but I'm not seeing the
  API to do this so virtio/qemu experts anyone know off-hand
  how to make this work? I started looking into the PCI callbacks
  reset() and virtio_device_ready() or possibly hitting the right
  set of bits with vp_set_status() but my first attempt just hung
  the device.

Signed-off-by: John Fastabend <redacted>
---

[...]

+
+	/* Changing the headroom in buffers is a disruptive operation because
+	 * existing buffers must be flushed and reallocated. This will happen
+	 * when a xdp program is initially added or xdp is disabled by removing
+	 * the xdp program.
+	 */
+	if (old_hr != vi->headroom) {
+		cancel_delayed_work_sync(&vi->refill);
+		if (netif_running(vi->dev)) {
+			for (i = 0; i < vi->max_queue_pairs; i++)
+				napi_disable(&vi->rq[i].napi);
+		}
+		for (i = 0; i < vi->max_queue_pairs; i++) {
+			struct virtqueue *vq = vi->rq[i].vq;
+			void *buf;
+
+			while ((buf = virtqueue_detach_unused_buf(vq)) != NULL) {

                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Per API in virtio_ring.c queue must be deactivated to call this.

+				if (vi->mergeable_rx_bufs) {
+					unsigned long ctx = (unsigned long)buf;
+					void *base = mergeable_ctx_to_buf_address(ctx);
+					put_page(virt_to_head_page(base));
+				} else if (vi->big_packets) {
+					give_pages(&vi->rq[i], buf);
+				} else {
+					dev_kfree_skb(buf);
+				}
+			}
+		}
+		if (netif_running(vi->dev)) {
+			for (i = 0; i < vi->max_queue_pairs; i++)
+				virtnet_napi_enable(&vi->rq[i]);
+		}
+	}
+

Hi Jason, Michael,

Any hints on how to solve the above kludge to flush the existing ring and
reallocate with correct headroom? The above doesn't look safe to me per commit
message.

Thanks!
John