Re: [PATCH v5 29/29] packet: use buf to store iovec array

[Laurent Vivier <lvivier@redhat.com>]

On 26/05/2025 16:21, Stefano Brivio wrote:
> On Thu, 17 Apr 2025 18:51:36 +0200
> Laurent Vivier <lvivier@redhat.com> wrote:
> 
>> When we use vhost-user we don't use the memory buffer
>> of the pool to store the packet, so we can use it to
>> store iovec array that points to the memory provided
>> by vhost-user.
>>
>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>> ---
>>   packet.c | 163 +++++++++++++++++++++++++++++++++++++++++++++++++------
>>   1 file changed, 147 insertions(+), 16 deletions(-)
>...
>> +/**
>> + * packet_iov_next_idx() - Give the the next available iovec index
>> + * @p:		Pointer to packet pool
>> + * @idx:	Index of packet descriptor in pool
>> + * @func:	For tracing: name of calling function
>> + * @line:	For tracing: caller line of function call
>> + *
>> + * Return: the next available iovec index
>> + */
>> +static size_t packet_iov_next_idx(const struct pool *p, size_t idx,
>> +				  const char *func, int line)
>> +{
>> +	size_t iov_idx, iov_cnt;
>> +
>> +	if (idx == 0)
>> +		return 0;
> 
> I'm a bit lost here. Why is 0 a special value now?

See below: we use "idx - 1"

> 
>> +
>> +	iov_idx = packet_iov_idx(p, idx - 1, &iov_cnt, func, line);
>> +
>> +	return iov_idx + iov_cnt;
>> +}
>> +

The next available iovec index for a given packet index is computed using the information 
from the previous packet index:

We take first available iovec index after the iovecs used by the previous packet.

The next available iovec index is the iovec index base of the previous packet plus all the 
iovecs used by the previous packet:

next available iovec index = (base iovec index of previous packet) + (number of iovec used 
by previous packet)

So, for the first packet (packet index 0), the first available iovec index is 0, we can't 
compute it from the previous packet because it doesn't exist (and all the iovec are free 
and available)

Thanks,
Laurent