ublk: update UBLK_F_SUPPORT_ZERO_COPY comment in UAPI header

UBLK_F_SUPPORT_ZERO_COPY has a very old comment describing the initial
idea for how zero-copy would be implemented. The actual implementation
added in commit 1f6540e ("ublk: zc register/unregister bvec") uses
io_uring registered buffers rather than shared memory mapping.
Remove the inaccurate remarks about mapping ublk request memory into the
ublk server's address space and requiring 4K block size. Replace them
with a description of the current zero-copy mechanism.

Signed-off-by: Caleb Sander Mateos <[email protected]>
Reviewed-by: Ming Lei <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: calebsander

include/uapi/linux/ublk_cmd.h

@@ -135,8 +135,28 @@
 #define UBLKSRV_IO_BUF_TOTAL_SIZE	(1ULL << UBLKSRV_IO_BUF_TOTAL_BITS)
 
 /*
- * zero copy requires 4k block size, and can remap ublk driver's io
- * request into ublksrv's vm space
+ * ublk server can register data buffers for incoming I/O requests with a sparse
+ * io_uring buffer table. The request buffer can then be used as the data buffer
+ * for io_uring operations via the fixed buffer index.
+ * Note that the ublk server can never directly access the request data memory.
+ *
+ * To use this feature, the ublk server must first register a sparse buffer
+ * table on an io_uring instance.
+ * When an incoming ublk request is received, the ublk server submits a
+ * UBLK_U_IO_REGISTER_IO_BUF command to that io_uring instance. The
+ * ublksrv_io_cmd's q_id and tag specify the request whose buffer to register
+ * and addr is the index in the io_uring's buffer table to install the buffer.
+ * SQEs can now be submitted to the io_uring to read/write the request's buffer
+ * by enabling fixed buffers (e.g. using IORING_OP_{READ,WRITE}_FIXED or
+ * IORING_URING_CMD_FIXED) and passing the registered buffer index in buf_index.
+ * Once the last io_uring operation using the request's buffer has completed,
+ * the ublk server submits a UBLK_U_IO_UNREGISTER_IO_BUF command with q_id, tag,
+ * and addr again specifying the request buffer to unregister.
+ * The ublk request is completed when its buffer is unregistered from all
+ * io_uring instances and the ublk server issues UBLK_U_IO_COMMIT_AND_FETCH_REQ.
+ *
+ * Not available for UBLK_F_UNPRIVILEGED_DEV, as a ublk server can leak
+ * uninitialized kernel memory by not reading into the full request buffer.
  */
 #define UBLK_F_SUPPORT_ZERO_COPY	(1ULL << 0)