Re: [PATCH v8 06/10] io_uring/rw: add support to send metadata along with read/write

[Christoph Hellwig <hch@lst.de>]

> +/*
> + * If sqe->ext_cap is set to this for IORING_OP_READ/WRITE, then the SQE
> + * contains protection information, and ring needs to be setup with SQE128
> + */
> +#define EXT_CAP_PI	(1U << 0)
> +
> +/* Second half of SQE128 for IORING_OP_READ/WRITE */
> +struct io_uring_sqe_ext {
> +	/*
> +	 * Reserved space for extended capabilities that are added down the
> +	 * line. Kept in beginning to maintain contiguity with the free space
> +	 * in first SQE
> +	 */
> +	__u64	rsvd0[4];

Thanks for documenting the design decision.  But I still don't understand
it.  Due to the layout it will be a bit hard to have fields spreading
form the "normal" SQE into the extended area anyway.  Note that this
is not a rejection of the approach, but I don't understand the argument
for it.

> +	/* only valid when EXT_CAP_PI is set */
> +	__u16	flags;
> +	__u16	pi_app_tag;
> +	__u32	pi_len;
> +	__u64	pi_addr;
> +	__u64	pi_seed;
> +	__u64	rsvd1;

.. but either way it would probably make sense to keep the reserved
areas together instead of spread out.

Otherwise this looks good to me.