Re: [PATCH v7 31/31] packet: use buf to store iovec array

[Stefano Brivio <sbrivio@redhat.com>]

On Mon, 23 Jun 2025 13:06:35 +0200
Laurent Vivier <lvivier@redhat.com> wrote:

> In vhost-user mode, where `pool->buf` doesn't store packet data directly,
> this patch repurposes it to hold the `struct iovec` array describing
> a scattered packet. This enables pools to manage true scatter-gather
> descriptors, with iovecs pointing to guest memory.
> 
> `p->pkt[idx].iov_base` is now an index into this `pool->buf` iovec array,
> and `p->pkt[idx].iov_len` is the count of iovecs. `packet_add_do` uses
> `iov_tail_slice` to store these iovec descriptors from an input iov_tail
> into `pool->buf`. `packet_data_do` reconstructs the iov_tail for a
> packet by pointing to this stored array of iovecs.
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  packet.c | 164 +++++++++++++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 148 insertions(+), 16 deletions(-)
> 
> diff --git a/packet.c b/packet.c
> index fd3b6db57c30..8dbe00af12c6 100644
> --- a/packet.c
> +++ b/packet.c
> @@ -84,6 +84,122 @@ bool pool_full(const struct pool *p)
>  	return p->count >= p->size;
>  }
>  
> +/**
> + * packet_iov_max_cnt() - Return the maximum number of iovec entries we can
> + * 			  store
> + * @p:		Pointer to packet pool
> + *
> + * Return: the maximum number of iovec entries we can store in the memory of
> + *         the pool buffer
> + */
> +static size_t packet_iov_max_cnt(const struct pool *p)
> +{
> +	return p->buf_size / sizeof(struct iovec);
> +}
> +
> +/**
> + * packet_iov_idx() - For a given packet index, return the iovec index and
> + * 		      the number of iovec entry of the packet
> + * @p:		Pointer to packet pool
> + * @idx:	Index of packet descriptor in pool
> + * @iov_cnt:	Pointer to store the number of the iovec entry of the packet
> + * @func:	For tracing: name of calling function
> + * @line:	For tracing: caller line of function call
> + *
> + * Return: the iovec index for the given packet index, @iov_cnt is set
> + *         to the number of the iovec entry of the packet
> + */
> +static int packet_iov_idx(const struct pool *p, size_t idx, size_t *iov_cnt,
> +			  const char *func, int line)
> +{
> +	size_t iov_idx, max = packet_iov_max_cnt(p);
> +
> +	iov_idx = (size_t)p->pkt[idx].iov_base;
> +	*iov_cnt = p->pkt[idx].iov_len;
> +
> +	ASSERT_WITH_MSG(iov_idx + *iov_cnt <= max,
> +		       "Corrupt iov entry: (%zu, %zu), max: %zu, %s:%i",
> +			iov_idx, *iov_cnt, max, func, line);
> +
> +	return iov_idx;
> +}
> +
> +/**
> + * packet_iov_next_idx() - Give the the next available iovec index

I'm taking the liberty to fix this comment while applying ("Get the next
available iovec index").

-- 
Stefano