Re: [PATCH v6 02/30] iov: Introduce iov_tail_clone() and iov_tail_drop().

[Stefano Brivio <sbrivio@redhat.com>]

On Wed,  4 Jun 2025 15:08:06 +0200
Laurent Vivier <lvivier@redhat.com> wrote:

> These utilities enhance iov_tail manipulation, useful for
> efficient packet processing by enabling iovec array cloning and
> header stripping without data copies.
> 
> - iov_tail_drop(): Discards a specified number of bytes from the
>   beginning of an iov_tail by advancing its internal offset and pruning
>   consumed elements.
> 
> - iov_tail_clone(): Clone an iov_tail into an iovec array, adjusting the
> first iovec entry to remove the iov_tail offset.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  iov.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  iov.h |  3 +++
>  2 files changed, 55 insertions(+)
> 
> diff --git a/iov.c b/iov.c
> index 91e87a740f0a..3b89f8126ee7 100644
> --- a/iov.c
> +++ b/iov.c
> @@ -192,6 +192,21 @@ size_t iov_tail_size(struct iov_tail *tail)
>  	return iov_size(tail->iov, tail->cnt) - tail->off;
>  }
>  
> +/**
> + * iov_tail_drop() - Discard a header from an IOV tail

Same as v5:

---
On Mon, 26 May 2025 16:19:03 +0200
Stefano Brivio <sbrivio@redhat.com> wrote:

> Mixing 'header' with 'tail' makes this a bit confusing. Yes, these
> vectors are used for headers, but what about calling it simply 'item',
> to match the context of this function?
---

> + * @tail:	IO vector tail
> + * @len:	length to move the head of the tail
> + *
> + * Returns:	true if the item still contains any bytes, otherwise false

Nit: we always use the imperative mood here, and no tabs, "Return: true
if...".

> + */
> +/* cppcheck-suppress unusedFunction */
> +bool iov_tail_drop(struct iov_tail *tail, size_t len)
> +{
> +	tail->off = tail->off + len;
> +
> +	return iov_tail_prune(tail);
> +}
> +
>  /**
>   * iov_peek_header_() - Get pointer to a header from an IOV tail
>   * @tail:	IOV tail to get header from
> @@ -248,3 +263,40 @@ void *iov_remove_header_(struct iov_tail *tail, size_t len, size_t align)
>  	tail->off = tail->off + len;
>  	return p;
>  }
> +
> +/**
> + * iov_tail_clone() -  Assign iov references referencing a subset of the data

Nit (same as for iov_slice() in v5): "iov_tail_clone() - Assign ..."

> + *		       in an iov_tail
> + *
> + * @dst_iov:     Pointer to the destination array of struct iovec describing
> + *		 the scatter/gather I/O vector to copy to.

Same as v5: perhaps "...to shallow copy to"?

> + * @dst_iov_cnt: Maximum number of elements in the destination iov array.
> + * @tail:	 Pointer to the source iov_tail
> + *
> + * Returns:      The number of elements successfully copied to the destination

"referenced from the destination iov array"?

Same as above: "Return: the number of..."

> + *		 iov array, a negative value if there is not enough room in the
> + *		 destination iov array
> + */
> +/* cppcheck-suppress unusedFunction */
> +ssize_t iov_tail_clone(struct iovec *dst_iov, size_t dst_iov_cnt,
> +		       struct iov_tail *tail)
> +{
> +	const struct iovec *iov = &tail->iov[0];
> +	size_t iov_cnt = tail->cnt;
> +	size_t offset = tail->off;
> +	unsigned int i, j;
> +
> +	i = iov_skip_bytes(iov, iov_cnt, offset, &offset);
> +
> +	/* assign iov references referencing a subset of the source one  */

Nit: excess whitespace.

> +	for (j = 0; i < iov_cnt && j < dst_iov_cnt; i++, j++) {
> +		dst_iov[j].iov_base = (char *)iov[i].iov_base + offset;
> +		dst_iov[j].iov_len = iov[i].iov_len - offset;
> +		offset = 0;
> +	}

Nice, this is much less scary than the previous iov_slice().

> +
> +	if (j == dst_iov_cnt && i != iov_cnt)
> +		return -1;
> +
> +	return j;
> +}
> diff --git a/iov.h b/iov.h
> index 9855bf0c0c32..11a8567aa183 100644
> --- a/iov.h
> +++ b/iov.h
> @@ -72,8 +72,11 @@ struct iov_tail {
>  
>  bool iov_tail_prune(struct iov_tail *tail);
>  size_t iov_tail_size(struct iov_tail *tail);
> +bool iov_tail_drop(struct iov_tail *tail, size_t len);
>  void *iov_peek_header_(struct iov_tail *tail, size_t len, size_t align);
>  void *iov_remove_header_(struct iov_tail *tail, size_t len, size_t align);
> +ssize_t iov_tail_clone(struct iovec *dst_iov, size_t dst_iov_cnt,
> +		       struct iov_tail *tail);
>  
>  /**
>   * IOV_PEEK_HEADER() - Get typed pointer to a header from an IOV tail

-- 
Stefano