RE: [PATCH net-next v9 2/2] xen networking: add XDP offset adjustment to xen-netback

[Paul Durrant <xadimgnik@gmail.com>]

> -----Original Message-----
> From: Denis Kirjanov <kda@linux-powerpc.org>
> Sent: 11 May 2020 11:22
> To: netdev@vger.kernel.org
> Cc: brouer@redhat.com; jgross@suse.com; wei.liu@kernel.org; paul@xen.org; ilias.apalodimas@linaro.org
> Subject: [PATCH net-next v9 2/2] xen networking: add XDP offset adjustment to xen-netback
> 
> the patch basically adds the offset adjustment and netfront
> state reading to make XDP work on netfront side.
> 
> Signed-off-by: Denis Kirjanov <denis.kirjanov@suse.com>
> ---
>  drivers/net/xen-netback/common.h  |  2 ++
>  drivers/net/xen-netback/netback.c |  7 +++++++
>  drivers/net/xen-netback/rx.c      |  7 ++++++-
>  drivers/net/xen-netback/xenbus.c  | 28 ++++++++++++++++++++++++++++
>  4 files changed, 43 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/xen-netback/common.h b/drivers/net/xen-netback/common.h
> index 05847eb..4a148d6 100644
> --- a/drivers/net/xen-netback/common.h
> +++ b/drivers/net/xen-netback/common.h
> @@ -280,6 +280,7 @@ struct xenvif {
>  	u8 ip_csum:1;
>  	u8 ipv6_csum:1;
>  	u8 multicast_control:1;
> +	u8 xdp_enabled:1;
> 
>  	/* Is this interface disabled? True when backend discovers
>  	 * frontend is rogue.
> @@ -395,6 +396,7 @@ static inline pending_ring_idx_t nr_pending_reqs(struct xenvif_queue *queue)
>  irqreturn_t xenvif_interrupt(int irq, void *dev_id);
> 
>  extern bool separate_tx_rx_irq;
> +extern bool provides_xdp_headroom;
> 
>  extern unsigned int rx_drain_timeout_msecs;
>  extern unsigned int rx_stall_timeout_msecs;
> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
> index 315dfc6..6dfca72 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -96,6 +96,13 @@
>  module_param_named(hash_cache_size, xenvif_hash_cache_size, uint, 0644);
>  MODULE_PARM_DESC(hash_cache_size, "Number of flows in the hash cache");
> 
> +/* The module parameter tells that we have to put data
> + * for xen-netfront with the XDP_PACKET_HEADROOM offset
> + * needed for XDP processing
> + */
> +bool provides_xdp_headroom = true;
> +module_param(provides_xdp_headroom, bool, 0644);
> +
>  static void xenvif_idx_release(struct xenvif_queue *queue, u16 pending_idx,
>  			       u8 status);
> 
> diff --git a/drivers/net/xen-netback/rx.c b/drivers/net/xen-netback/rx.c
> index ef58870..c97c98e 100644
> --- a/drivers/net/xen-netback/rx.c
> +++ b/drivers/net/xen-netback/rx.c
> @@ -33,6 +33,11 @@
>  #include <xen/xen.h>
>  #include <xen/events.h>
> 
> +static int xenvif_rx_xdp_offset(struct xenvif *vif)
> +{
> +	return vif->xdp_enabled ? XDP_PACKET_HEADROOM : 0;
> +}
> +
>  static bool xenvif_rx_ring_slots_available(struct xenvif_queue *queue)
>  {
>  	RING_IDX prod, cons;
> @@ -356,7 +361,7 @@ static void xenvif_rx_data_slot(struct xenvif_queue *queue,
>  				struct xen_netif_rx_request *req,
>  				struct xen_netif_rx_response *rsp)
>  {
> -	unsigned int offset = 0;
> +	unsigned int offset = xenvif_rx_xdp_offset(queue->vif);
>  	unsigned int flags;
> 
>  	do {
> diff --git a/drivers/net/xen-netback/xenbus.c b/drivers/net/xen-netback/xenbus.c
> index 286054b..d447191 100644
> --- a/drivers/net/xen-netback/xenbus.c
> +++ b/drivers/net/xen-netback/xenbus.c
> @@ -393,6 +393,20 @@ static void set_backend_state(struct backend_info *be,
>  	}
>  }
> 
> +static void read_xenbus_frontend_xdp(struct backend_info *be,
> +				     struct xenbus_device *dev)
> +{
> +	struct xenvif *vif = be->vif;
> +	unsigned int val;
> +	int err;
> +
> +	err = xenbus_scanf(XBT_NIL, dev->otherend,
> +			   "feature-xdp", "%u", &val);
> +	if (err != 1)
> +		return;
> +	vif->xdp_enabled = val;
> +}
> +
>  /**
>   * Callback received when the frontend's state changes.
>   */
> @@ -417,6 +431,11 @@ static void frontend_changed(struct xenbus_device *dev,
>  		set_backend_state(be, XenbusStateConnected);
>  		break;
> 
> +	case XenbusStateReconfiguring:
> +		read_xenbus_frontend_xdp(be, dev);

I think this being the only call to read_xenbus_frontend_xdp() is still a problem. What happens if netback is reloaded against a
netfront that has already enabled 'feature-xdp'? AFAICT vif->xdp_enabled would remain false after the reload.

  Paul

> +		xenbus_switch_state(dev, XenbusStateReconfigured);
> +		break;
> +
>  	case XenbusStateClosing:
>  		set_backend_state(be, XenbusStateClosing);
>  		break;
> @@ -1036,6 +1055,15 @@ static int netback_probe(struct xenbus_device *dev,
>  			goto abort_transaction;
>  		}
> 
> +		/* we can adjust a headroom for netfront XDP processing */
> +		err = xenbus_printf(xbt, dev->nodename,
> +				    "feature-xdp-headroom", "%d",
> +				    provides_xdp_headroom);
> +		if (err) {
> +			message = "writing feature-xdp-headroom";
> +			goto abort_transaction;
> +		}
> +
>  		/* We don't support rx-flip path (except old guests who
>  		 * don't grok this feature flag).
>  		 */
> --
> 1.8.3.1