Re: [RFC 1/8] virtio: Define InOrderVQElement

[Jonah Palmer]

On 3/22/24 5:45 AM, Eugenio Perez Martin wrote:
> On Thu, Mar 21, 2024 at 4:57 PM Jonah Palmer <jonah.palmer@oracle.com> wrote:
> > Define the InOrderVQElement structure for the VIRTIO_F_IN_ORDER
> > transport feature implementation.
> >
> > The InOrderVQElement structure is used to encapsulate out-of-order
> > VirtQueueElement data that was processed by the host. This data
> > includes:
> >   - The processed VirtQueueElement (elem)
> >   - Length of data (len)
> >   - VirtQueueElement array index (idx)
> >   - Number of processed VirtQueueElements (count)
> >
> > InOrderVQElements will be stored in a buffering mechanism until an
> > order can be achieved.
> >
> > Signed-off-by: Jonah Palmer <jonah.palmer@oracle.com>
> > ---
> >   include/hw/virtio/virtio.h | 7 +++++++
> >   1 file changed, 7 insertions(+)
> >
> > diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> > index b3c74a1bca..c8aa435a5e 100644
> > --- a/include/hw/virtio/virtio.h
> > +++ b/include/hw/virtio/virtio.h
> > @@ -77,6 +77,13 @@ typedef struct VirtQueueElement
> >       struct iovec *out_sg;
> >   } VirtQueueElement;
> >
> > +typedef struct InOrderVQElement {
> > +    const VirtQueueElement *elem;
>
> Some subsystems allocate space for extra elements after
> VirtQueueElement, like VirtIOBlockReq. You can request virtqueue_pop
> to allocate this extra space by its second argument. Would it work for
> this?

I don't see why not. Although this may not be necessary due to me 
missing a key aspect mentioned in your comment below.

> > +    unsigned int len;
> > +    unsigned int idx;
> > +    unsigned int count;
>
> Now I don't get why these fields cannot be obtained from elem->(len,
> index, ndescs) ?

Interesting. I didn't realize that these values are equivalent to a 
VirtQueueElement's len, index, and ndescs fields.

Is this always true? Else I would've expected, for example, 
virtqueue_push to not need the 'unsigned int len' parameter if this 
information is already included via. the VirtQueueElement being passed in.

> > +} InOrderVQElement;
> > +
> >   #define VIRTIO_QUEUE_MAX 1024
> >
> >   #define VIRTIO_NO_VECTOR 0xffff
> > --
> > 2.39.3