Subject: Re: [net-next PATCH v5 5/6] virtio_net: add XDP_TX support

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 8 Dec 2016 23:08:07 +0200

On Thu, Dec 08, 2016 at 10:18:22AM -0800, John Fastabend wrote:
> On 16-12-07 10:11 PM, Michael S. Tsirkin wrote:
> > On Wed, Dec 07, 2016 at 12:12:45PM -0800, John Fastabend wrote:
> >> This adds support for the XDP_TX action to virtio_net. When an XDP
> >> program is run and returns the XDP_TX action the virtio_net XDP
> >> implementation will transmit the packet on a TX queue that aligns
> >> with the current CPU that the XDP packet was processed on.
> >>
> >> Before sending the packet the header is zeroed.  Also XDP is expected
> >> to handle checksum correctly so no checksum offload  support is
> >> provided.
> >>
> >> Signed-off-by: John Fastabend <[email-masked]>
> >> ---
> >>  drivers/net/virtio_net.c |   99 +++++++++++++++++++++++++++++++++++++++++++---
> >>  1 file changed, 92 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> >> index 28b1196..8e5b13c 100644
> >> --- a/drivers/net/virtio_net.c
> >> +++ b/drivers/net/virtio_net.c
> >> @@ -330,12 +330,57 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
> >>  	return skb;
> >>  }
> >>  
> >> +static void virtnet_xdp_xmit(struct virtnet_info *vi,
> >> +			     struct receive_queue *rq,
> >> +			     struct send_queue *sq,
> >> +			     struct xdp_buff *xdp)
> >> +{
> >> +	struct page *page = virt_to_head_page(xdp->data);
> >> +	struct virtio_net_hdr_mrg_rxbuf *hdr;
> >> +	unsigned int num_sg, len;
> >> +	void *xdp_sent;
> >> +	int err;
> >> +
> >> +	/* Free up any pending old buffers before queueing new ones. */
> >> +	while ((xdp_sent = virtqueue_get_buf(sq->vq, &len)) != NULL) {
> >> +		struct page *sent_page = virt_to_head_page(xdp_sent);
> >> +
> >> +		if (vi->mergeable_rx_bufs)
> >> +			put_page(sent_page);
> >> +		else
> >> +			give_pages(rq, sent_page);
> >> +	}
> > 
> > Looks like this is the only place where you do virtqueue_get_buf.
> > No interrupt handler?
> > This means that if you fill up the queue, nothing will clean it
> > and things will get stuck.
> 
> hmm OK so the callbacks should be implemented to do this and a pair
> of virtqueue_enable_cb_prepare()/virtqueue_disable_cb() used to enable
> and disable callbacks if packets are enqueued.

Oh I didn't realize XDP never stops processing packets,
even if they are never freed.
In that case you do not need callbacks.

> Also in the normal xmit path via start_xmit() will the same condition
> happen? It looks like free_old_xmit_skbs for example is only called if
> a packet is sent could we end up holding on to skbs in this case? I
> don't see free_old_xmit_skbs being called from any callbacks?

Right - all it does is restart the queue. That's why we don't support
BQL right now.

> > Can this be the issue you saw?
> 
> nope see below I was mishandling the big_packets page cleanup path in
> the error case.
> 
> > 
> > 
> >> +
> >> +	/* Zero header and leave csum up to XDP layers */
> >> +	hdr = xdp->data;
> >> +	memset(hdr, 0, vi->hdr_len);
> >> +
> >> +	nu_sg = 1;
> >> +	sg_init_one(sq->sg, xdp->data, xdp->data_end - xdp->data);
> >> +	err = virtqueue_add_outbuf(sq->vq, sq->sg, num_sg,
> >> +				   xdp->data, GFP_ATOMIC);
> >> +	if (unlikely(err)) {
> >> +		if (vi->mergeable_rx_bufs)
> >> +			put_page(page);
> >> +		else
> >> +			give_pages(rq, page);
> >> +	} else if (!vi->mergeable_rx_bufs) {
> >> +		/* If not mergeable bufs must be big packets so cleanup pages */
> >> +		give_pages(rq, (struct page *)page->private);
> >> +		page->private = 0;
> >> +	}
> >> +
> >> +	virtqueue_kick(sq->vq);
> > 
> > Is this unconditional kick a work-around for hang
> > we could not figure out yet?
> 
> I tracked the original issue down to how I handled the big_packet page
> cleanups.
> 
> > I guess this helps because it just slows down the guest.
> > I don't much like it ...
> 
> I left it like this copying the pattern in balloon and input drivers. I
> can change it back to the previous pattern where it is only called if
> there is no errors. It has been running fine with the old pattern now
> for an hour or so.
> 
> .John

OK makes sense.