Re: [PATCH v2 3/3] vu_common: Move iovec management into vu_collect()

[Laurent Vivier <lvivier@redhat.com>]

On 3/17/26 16:23, Stefano Brivio wrote:
> On Tue, 17 Mar 2026 08:25:49 +0100
> Laurent Vivier <lvivier@redhat.com> wrote:
> 
>> On 3/17/26 03:40, David Gibson wrote:
>>> On Fri, Mar 13, 2026 at 07:26:18PM +0100, Laurent Vivier wrote:
>>>> Previously, callers had to pre-initialize virtqueue elements with iovec
>>>> entries using vu_set_element() or vu_init_elem() before calling
>>>> vu_collect().  This meant each element owned a fixed, pre-assigned iovec
>>>> slot.
>>>>
>>>> Move the iovec array into vu_collect() as explicit parameters (in_sg,
>>>> max_in_sg, and in_num), letting it pass the remaining iovec capacity
>>>> directly to vu_queue_pop().  A running current_iov counter tracks
>>>> consumed entries across elements, so multiple elements share a single
>>>> iovec pool.  The optional in_num output parameter reports how many iovec
>>>> entries were consumed, allowing callers to track usage across multiple
>>>> vu_collect() calls.
>>>>
>>>> This removes vu_set_element() and vu_init_elem() which are no longer
>>>> needed, and is a prerequisite for multi-buffer support where a single
>>>> virtqueue element can use more than one iovec entry.  For now, callers
>>>> assert the current single-iovec-per-element invariant until they are
>>>> updated to handle multiple iovecs.
>>>>
>>>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
>>>
>>> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
>>>
>>> Couple of thoughts on possible polish below.
>>>
>>> [snip]
>>>>    /**
>>>>     * vu_collect() - collect virtio buffers from a given virtqueue
>>>>     * @vdev:		vhost-user device
>>>>     * @vq:			virtqueue to collect from
>>>> - * @elem:		Array of virtqueue element
>>>> - * 			each element must be initialized with one iovec entry
>>>> - * 			in the in_sg array.
>>>> + * @elem:		Array of @max_elem virtqueue elements
>>>>     * @max_elem:		Number of virtqueue elements in the array
>>>> + * @in_sg:		Incoming iovec array for device-writable descriptors
>>>> + * @max_in_sg:		Maximum number of entries in @in_sg
>>>> + * @in_num:		Number of collected entries from @in_sg (output)
>>>>     * @size:		Maximum size of the data in the frame
>>>>     * @collected:		Collected buffer length, up to @size, set on return
>>>>     *
>>>> @@ -80,20 +67,21 @@ void vu_init_elem(struct vu_virtq_element *elem, struct iovec *iov, int elem_cnt
>>>>     */
>>>>    int vu_collect(const struct vu_dev *vdev, struct vu_virtq *vq,
>>>>    	       struct vu_virtq_element *elem, int max_elem,
>>>> +	       struct iovec *in_sg, size_t max_in_sg, size_t *in_num,
>>>>    	       size_t size, size_t *collected)
>>>>    {
>>>>    	size_t current_size = 0;
>>>> +	size_t current_iov = 0;
>>>>    	int elem_cnt = 0;
>>>>    
>>>> -	while (current_size < size && elem_cnt < max_elem) {
>>>> -		struct iovec *iov;
>>>> +	while (current_size < size && elem_cnt < max_elem &&
>>>> +	       current_iov < max_in_sg) {
>>>>    		int ret;
>>>>    
>>>>    		ret = vu_queue_pop(vdev, vq, &elem[elem_cnt],
>>>> -				   elem[elem_cnt].in_sg,
>>>> -				   elem[elem_cnt].in_num,
>>>> -				   elem[elem_cnt].out_sg,
>>>> -				   elem[elem_cnt].out_num);
>>>> +				   &in_sg[current_iov],
>>>> +				   max_in_sg - current_iov,
>>>> +				   NULL, 0);
>>>>    		if (ret < 0)
>>>>    			break;
>>>>    
>>>> @@ -103,18 +91,22 @@ int vu_collect(const struct vu_dev *vdev, struct vu_virtq *vq,
>>>>    			break;
>>>>    		}
>>>>    
>>>> -		iov = &elem[elem_cnt].in_sg[0];
>>>> -
>>>> -		if (iov->iov_len > size - current_size)
>>>> -			iov->iov_len = size - current_size;
>>>> +		elem[elem_cnt].in_num = iov_truncate(elem[elem_cnt].in_sg,
>>>> +						     elem[elem_cnt].in_num,
>>>> +						     size - current_size);
>>>
>>> Will elem[].in_num always end up with the same value as the @in_num
>>> parameter?  If so, do we need the explicit parameter?
>>
>> @in_num parameter of vu_collect()?
>>
>> @in_num is the sum of all elem[].in_num, it can be computed by the caller function from
>> elem, but it is simpler to return it as we need to compute it in the loop.
> 
> I'm not sure I understood the point of David's comment here, and this
> explanation makes sense to me now, but it took me a bit to figure that
> out.
> 
> Could it be that @in_num is a bit confusing as it has "in" and "num" in
> it, but it's actually an output representing how many "in" entries we
> used/need?

For an element, *ìn_*num is the number of *in_*sg we have read from the ring for an element.

It's virtio semantic, so  *in_* means sg going *in* the guest.

For *out_*sg we have *out_*num.

> 
> What if we rename it to @in_used or @in_collected?
> 

The idea was to keep the same name as in the element. But we can change this to @in_used.

Thanks,
Laurent