[PATCH net-next 12/33] hv_netvsc: add XDP frame size to driver

[Jesper Dangaard Brouer <brouer@redhat.com>]

The hyperv NIC drivers XDP implementation is rather disappointing as it
will be a slowdown to enable XDP on this driver, given it will allocate a
new page for each packet and copy over the payload, before invoking the
XDP BPF-prog.

The positive thing it that its easy to determine the xdp.frame_sz.

The XDP implementation for hv_netvsc transparently passes xdp_prog
to the associated VF NIC. Many of the Azure VMs are using SRIOV, so
majority of the data are actually processed directly on the VF driver's XDP
path. So the overhead of the synthetic data path (hv_netvsc) is minimal.

Then XDP is enabled on this driver, XDP_PASS and XDP_TX will create the
SKB via build_skb (based on the newly allocated page). Now using XDP
frame_sz this will provide more skb_tailroom, which netstack can use for
SKB coalescing (e.g tcp_try_coalesce -> skb_try_coalesce).

Cc: Wei Liu <wei.liu@kernel.org>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 drivers/net/hyperv/netvsc_bpf.c |    1 +
 drivers/net/hyperv/netvsc_drv.c |    2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/hyperv/netvsc_bpf.c b/drivers/net/hyperv/netvsc_bpf.c
index b86611041db6..1e0c024b0a93 100644
--- a/drivers/net/hyperv/netvsc_bpf.c
+++ b/drivers/net/hyperv/netvsc_bpf.c
@@ -49,6 +49,7 @@ u32 netvsc_run_xdp(struct net_device *ndev, struct netvsc_channel *nvchan,
 	xdp_set_data_meta_invalid(xdp);
 	xdp->data_end = xdp->data + len;
 	xdp->rxq = &nvchan->xdp_rxq;
+	xdp->frame_sz = PAGE_SIZE;
 	xdp->handle = 0;
 
 	memcpy(xdp->data, data, len);
diff --git a/drivers/net/hyperv/netvsc_drv.c b/drivers/net/hyperv/netvsc_drv.c
index d8e86bdbfba1..651344fea0a5 100644
--- a/drivers/net/hyperv/netvsc_drv.c
+++ b/drivers/net/hyperv/netvsc_drv.c
@@ -794,7 +794,7 @@ static struct sk_buff *netvsc_alloc_recv_skb(struct net_device *net,
 	if (xbuf) {
 		unsigned int hdroom = xdp->data - xdp->data_hard_start;
 		unsigned int xlen = xdp->data_end - xdp->data;
-		unsigned int frag_size = netvsc_xdp_fraglen(hdroom + xlen);
+		unsigned int frag_size = xdp->frame_sz;
 
 		skb = build_skb(xbuf, frag_size);