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

On Wed, 4 Jun 2025 15:08:06 +0200 Laurent Vivier 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 --- 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 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