[PATCH v2 25/25] docs: fuse: add io-uring bufring and zero-copy documentation

[Joanne Koong <joannelkoong@gmail.com>]

Add documentation for fuse over io-uring usage of kernel-managed
bufrings and zero-copy.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
---
 .../filesystems/fuse/fuse-io-uring.rst        | 55 ++++++++++++++++++-
 1 file changed, 54 insertions(+), 1 deletion(-)

diff --git a/Documentation/filesystems/fuse/fuse-io-uring.rst b/Documentation/filesystems/fuse/fuse-io-uring.rst
index d73dd0dbd238..4c17169069e9 100644
--- a/Documentation/filesystems/fuse/fuse-io-uring.rst
+++ b/Documentation/filesystems/fuse/fuse-io-uring.rst
@@ -95,5 +95,58 @@ Sending requests with CQEs
  |    <fuse_unlink()                         |
  |  <sys_unlink()                            |
 
+Kernel-managed buffer rings
+===========================
 
-
+Kernel-managed buffer rings have two main advantages:
+* eliminates the overhead of pinning/unpinning user pages and translating
+  virtual addresses for every server-kernel interaction
+* reduces buffer memory allocation requirements
+
+In order to use buffer rings, the server must preregister the following:
+* a fixed buffer at index 0. This is where the headers will reside
+* a kernel-managed buffer ring. This is where the payload will reside
+
+At a high-level, this is how fuse uses buffer rings:
+* The server registers a kernel-managed buffer ring. In the kernel this
+  allocates the pages needed for the buffers and vmaps them. The server
+  obtains the virtual address for the buffers through an mmap call on the ring
+  fd.
+* When there is a request from a client, fuse will select a buffer from the
+  ring if there is any payload that needs to be copied, copy over the payload
+  to the selected buffer, and copy over the headers to the fixed buffer at
+  index 0, at the buffer id that corresponds to the server (which the server
+  needs to specify through sqe->buf_index).
+* The server obtains a cqe representing the request. The cqe flag will have
+  IORING_CQE_F_BUFFER set if a selected buffer was used for the payload. The
+  buffer id is stashed in cqe->flags (through IORING_CQE_BUFFER_SHIFT). The
+  server can directly access the payload by using that buffer id to calculate
+  the offset into the virtual address obtained for the buffers.
+* The server processes the request and then sends a
+  FUSE_URING_CMD_COMMIT_AND_FETCH sqe with the reply.
+* When the kernel handles the sqe, it will process the reply and if there is a
+  next request, it will reuse the same selected buffer for the request. If
+  there is no next request, it will recycle the buffer back to the ring.
+
+Zero-copy
+=========
+
+Fuse io-uring zero-copy allows the server to directly read from / write to the
+client's pages and bypass any intermediary buffer copies. This is only allowed
+on privileged servers.
+
+In order to use zero-copy, the server must pregister the following:
+* a sparse buffer for every entry in the queue. This is where the client's
+  pages will reside
+* a fixed buffer at index queue_depth (tailing the sparse buffer).
+  This is where the headers will reside
+* a kernel-managed buffer ring. This is where any non-zero-copied payload (eg
+  out headers) will reside
+
+When the client issues a read/write, fuse stores the client's underlying pages
+in the sparse buffer entry corresponding to the ent in the queue. The server
+can then issue reads/writes on these pages through io_uring rw operations.
+Please note that the server is not able to directly access these pages, it
+must go through the io-uring interface to read/write to them. The pages are
+unregistered once the server replies to the request. Non-zero-copyable
+payload (if needed) is placed in a buffer from the kernel-managed buffer ring.
-- 
2.47.3