[PATCH 5/7] io_uring: add ability for provided buffer to index registered buffers

[Jens Axboe <axboe@kernel.dk>]

This just adds the necessary shifts that define what a provided buffer
that is merely an index into a registered buffer looks like. A provided
buffer looks like the following:

struct io_uring_buf {
	__u64	addr;
	__u32	len;
	__u16	bid;
	__u16	resv;
};

where 'addr' holds a userspace address, 'len' is the length of the
buffer, and 'bid' is the buffer ID identifying the buffer. This works
fine for a virtual address, but it cannot be used efficiently denote
a registered buffer. Registered buffers are pre-mapped into the kernel
for more efficient IO, avoiding a get_user_pages() and page(s) inc+dec,
and are used for things like O_DIRECT on storage and zero copy send.

Particularly for the send case, it'd be useful to support a mix of
provided and registered buffers. This enables the use of using a
provided ring buffer to serialize sends, and also enables the use of
send bundles, where a send can pick multiple buffers and send them all
at once.

If provided buffers are used as an index into registered buffers, the
meaning of buf->addr changes. If registered buffer index 'regbuf_index'
is desired, with a length of 'len' and the offset 'regbuf_offset' from
the start of the buffer, then the application would fill out the entry
as follows:

buf->addr = ((__u64) regbuf_offset << IOU_BUF_OFFSET_BITS) | regbuf_index;
buf->len = len;

and otherwise add it to the buffer ring as usual. The kernel will then
first pick a buffer from the desired buffer group ID, and then decode
which registered buffer to use for the transfer.

This provides a way to use both registered and provided buffers at the
same time.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/uapi/linux/io_uring.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 86cb385fe0b5..eef88d570cb4 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -733,6 +733,14 @@ struct io_uring_buf_ring {
 	};
 };
 
+/*
+ * When provided buffers are used as indices into registered buffers, the
+ * lower IOU_BUF_REGBUF_BITS indicate the index into the registered buffers,
+ * and the upper IOU_BUF_OFFSET_BITS indicate the offset into that buffer.
+ */
+#define IOU_BUF_REGBUF_BITS	(32ULL)
+#define IOU_BUF_OFFSET_BITS	(32ULL)
+
 /*
  * Flags for IORING_REGISTER_PBUF_RING.
  *
-- 
2.45.2