ntb_netdev: Use dev_kfree_skb_any() in interrupt context

In the Linux kernel, the following vulnerability has been resolved:

ntb_netdev: Use dev_kfree_skb_any() in interrupt context

TX/RX callback handlers (ntb_netdev_tx_handler(),

ntb_netdev_rx_handler()) can be called in interrupt

context via the DMA framework when the respective

DMA operations have completed. As such, any calls

by these routines to free skb's, should use the

interrupt context safe dev_kfree_skb_any() function.

Previously, these callback handlers would call the

interrupt unsafe version of dev_kfree_skb(). This has

not presented an issue on Intel IOAT DMA engines as

that driver utilizes tasklets rather than a hard

interrupt handler, like the AMD PTDMA DMA driver.

On AMD systems, a kernel WARNING message is

encountered, which is being issued from

skb_release_head_state() due to in_hardirq()

being true.

Besides the user visible WARNING from the kernel,

the other symptom of this bug was that TCP/IP performance

across the ntb_netdev interface was very poor, i.e.

approximately an order of magnitude below what was

expected. With the repair to use dev_kfree_skb_any(),

kernel WARNINGs from skb_release_head_state() ceased

and TCP/IP performance, as measured by iperf, was on

par with expected results, approximately 20 Gb/s on

AMD Milan based server. Note that this performance

is comparable with Intel based servers.