Re: [PATCH RFC v2 01/33] xdp: add frame size to xdp_buff

On Wed, 2020-04-08 at 10:53 -0700, Jakub Kicinski wrote:

On Wed, 08 Apr 2020 13:50:39 +0200 Jesper Dangaard Brouer wrote:

quoted

XDP have evolved to support several frame sizes, but xdp_buff was
not
updated with this information. The frame size (frame_sz) member of
xdp_buff is introduced to know the real size of the memory the
frame is
delivered in.

When introducing this also make it clear that some tailroom is
reserved/required when creating SKBs using build_skb().

It would also have been an option to introduce a pointer to
data_hard_end (with reserved offset). The advantage with frame_sz
is
that (like rxq) drivers only need to setup/assign this value once
per
NAPI cycle. Due to XDP-generic (and some drivers) it's not possible
to
store frame_sz inside xdp_rxq_info, because it's varies per packet
as it
can be based/depend on packet length.

Signed-off-by: Jesper Dangaard Brouer <redacted>
---
 include/net/xdp.h |   17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/include/net/xdp.h b/include/net/xdp.h
index 40c6d3398458..99f4374f6214 100644
--- a/include/net/xdp.h
+++ b/include/net/xdp.h

@@ -6,6 +6,8 @@
 #ifndef __LINUX_NET_XDP_H__
 #define __LINUX_NET_XDP_H__
 
+#include <linux/skbuff.h> /* skb_shared_info */
+
 /**
  * DOC: XDP RX-queue information
  *

@@ -70,8 +72,23 @@ struct xdp_buff {
 	void *data_hard_start;
 	unsigned long handle;
 	struct xdp_rxq_info *rxq;
+	u32 frame_sz; /* frame size to deduct data_hard_end/reserved

tailroom*/

Perhaps

/* length of packet buffer, starting at data_hard_start */

?

quoted

 };
 
+/* Reserve memory area at end-of data area.

I wouldn't say this reserves anything. It just computes the end
pointer, no?

quoted

+ *
+ * This macro reserves tailroom in the XDP buffer by limiting the
+ * XDP/BPF data access to data_hard_end.  Notice same area (and
size)
+ * is used for XDP_PASS, when constructing the SKB via
build_skb().
+ */
+#define xdp_data_hard_end(xdp)				\
+	((xdp)->data_hard_start + (xdp)->frame_sz -	\
+	 SKB_DATA_ALIGN(sizeof(struct skb_shared_info)))

I think it should be said somewhere that the drivers are expected to
DMA map memory up to xdp_data_hard_end(xdp).

but this works on a specific xdp buff, drivers work with mtu

and what if the driver want to have this as an option per packet .. 
i.e.: if there is enough tail room, then build_skb, otherwise
alloc new skb, copy headers, setup data frags.. etc

having such limitations on driver can be very strict, i think the
decision must remain dynamic per frame..

of-course drivers should optimize to preserve enough tail room for all
rx packets.. 

quoted

+
+/* Like skb_shinfo */
+#define xdp_shinfo(xdp)	((struct skb_shared_info
*)(xdp_data_hard_end(xdp)))
+// XXX: Above likely belongs in later patch
+
 struct xdp_frame {
 	void *data;
 	u16 len;