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>

1 files changed, 5 insertions, 7 deletions

diff --git a/include/uapi/linux/if_xdp.h b/include/uapi/linux/if_xdp.h
index 4737cfe222f5..e411d6f9ac65 100644
--- a/include/uapi/linux/if_xdp.h
+++ b/include/uapi/linux/if_xdp.h
@@ -48,8 +48,8 @@ struct xdp_mmap_offsets {
 struct xdp_umem_reg {
         __u64 addr; /* Start of packet data area */
         __u64 len; /* Length of packet data area */
-        __u32 frame_size; /* Frame size */
-        __u32 frame_headroom; /* Frame head room */
+        __u32 chunk_size;
+        __u32 headroom;
 };
 
 struct xdp_statistics {
@@ -66,13 +66,11 @@ struct xdp_statistics {
 
 /* Rx/Tx descriptor */
 struct xdp_desc {
-        __u32 idx;
+        __u64 addr;
         __u32 len;
-        __u16 offset;
-        __u8 flags;
-        __u8 padding[5];
+        __u32 options;
 };
 
-/* UMEM descriptor is __u32 */
+/* UMEM descriptor is __u64 */
 
 #endif /* _LINUX_IF_XDP_H */