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

[Pavel Begunkov <asml.silence@gmail.com>]

On 10/23/24 17:07, Jens Axboe wrote:
> 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)

32 bit is fine for IO size but not enough to store offsets, it
can only address under 4GB registered buffers.


-- 
Pavel Begunkov