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xsk: new descriptor addressing scheme

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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>
This commit is contained in:
Björn Töpel
2018-06-04 13:57:13 +02:00
committed by Daniel Borkmann
parent a509a95536
commit bbff2f321a
8 changed files with 123 additions and 129 deletions
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12
include/uapi/linux/if_xdp.h
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@@ -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 */