xsk: new descriptor addressing scheme

Currently, AF_XDP only supports a fixed frame-size memory scheme where
each frame is referenced via an index (idx). A user passes the frame
index to the kernel, and the kernel acts upon the data.  Some NICs,
however, do not have a fixed frame-size model, instead they have a
model where a memory window is passed to the hardware and multiple
frames are filled into that window (referred to as the "type-writer"
model).

By changing the descriptor format from the current frame index
addressing scheme, AF_XDP can in the future be extended to support
these kinds of NICs.

In the index-based model, an idx refers to a frame of size
frame_size. Addressing a frame in the UMEM is done by offseting the
UMEM starting address by a global offset, idx * frame_size + offset.
Communicating via the fill- and completion-rings are done by means of
idx.

In this commit, the idx is removed in favor of an address (addr),
which is a relative address ranging over the UMEM. To convert an
idx-based address to the new addr is simply: addr = idx * frame_size +
offset.

We also stop referring to the UMEM "frame" as a frame. Instead it is
simply called a chunk.

To transfer ownership of a chunk to the kernel, the addr of the chunk
is passed in the fill-ring. Note, that the kernel will mask addr to
make it chunk aligned, so there is no need for userspace to do
that. E.g., for a chunk size of 2k, passing an addr of 2048, 2050 or
3000 to the fill-ring will refer to the same chunk.

On the completion-ring, the addr will match that of the Tx descriptor,
passed to the kernel.

Changing the descriptor format to use chunks/addr will allow for
future changes to move to a type-writer based model, where multiple
frames can reside in one chunk. In this model passing one single chunk
into the fill-ring, would potentially result in multiple Rx
descriptors.

This commit changes the uapi of AF_XDP sockets, and updates the
documentation.

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/xdp/xdp_umem.h

@@ -18,35 +18,20 @@ struct xdp_umem {
 	struct xsk_queue *cq;
 	struct page **pgs;
 	struct xdp_umem_props props;
-	u32 npgs;
-	u32 frame_headroom;
-	u32 nfpp_mask;
-	u32 nfpplog2;
-	u32 frame_size_log2;
+	u32 headroom;
+	u32 chunk_size_nohr;
 	struct user_struct *user;
 	struct pid *pid;
 	unsigned long address;
-	size_t size;
 	refcount_t users;
 	struct work_struct work;
+	u32 npgs;
 };
 
-static inline char *xdp_umem_get_data(struct xdp_umem *umem, u32 idx)
+static inline char *xdp_umem_get_data(struct xdp_umem *umem, u64 addr)
 {
-	u64 pg, off;
-	char *data;
-
-	pg = idx >> umem->nfpplog2;
-	off = (idx & umem->nfpp_mask) << umem->frame_size_log2;
-
-	data = page_address(umem->pgs[pg]);
-	return data + off;
-}
-
-static inline char *xdp_umem_get_data_with_headroom(struct xdp_umem *umem,
-						    u32 idx)
-{
-	return xdp_umem_get_data(umem, idx) + umem->frame_headroom;
+	return page_address(umem->pgs[addr >> PAGE_SHIFT]) +
+		(addr & (PAGE_SIZE - 1));
 }
 
 bool xdp_umem_validate_queues(struct xdp_umem *umem);