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

[Laurent Vivier <lvivier@redhat.com>]

On 3/18/26 10:04, Stefano Brivio wrote:
> On Wed, 18 Mar 2026 08:21:10 +0100
> Laurent Vivier <lvivier@redhat.com> wrote:
> 
>> On 3/18/26 02:16, David Gibson wrote:
>>> On Tue, Mar 17, 2026 at 05:30:32PM +0100, Laurent Vivier wrote:
>>>> 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.
>>>
>>> Would "in_total" work better to suggest that it's the sum of all the
>>> elements' in_num?
>>
>> Yes, I think it gives the information it's the sum of the in_num
> 
> Okay, should I change this on merge, or do you plan to repost
> (which would be slightly more convenient for me but not really needed)?
> 

I'm preparing the v3.

Thanks,
Laurent