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

[Stefano Brivio <sbrivio@redhat.com>]

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?

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

-- 
Stefano