linux-kernel - Re: [PATCH V7 12/19] virtio_ring: use u16 for last_used_idx in virtqueue_poll

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Message-ID: <20250929041808-mutt-send-email-mst@kernel.org>
Date: Mon, 29 Sep 2025 04:22:16 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: David Laight <david.laight.linux@...il.com>
Cc: Jason Wang <jasowang@...hat.com>, xuanzhuo@...ux.alibaba.com,
	eperezma@...hat.com, virtualization@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V7 12/19] virtio_ring: use u16 for last_used_idx in
 virtqueue_poll_split()

On Sun, Sep 28, 2025 at 07:27:19PM +0100, David Laight wrote:
> On Thu, 25 Sep 2025 18:37:01 +0800
> Jason Wang <jasowang@...hat.com> wrote:
> 
> > Use u16 for last_used_idx in virtqueue_poll_split() to align with the
> > spec.
> 
> If you care about performance you should pretty much never use 'u16' for
> function parameters, return values or any arithmetic.
> Just because the domain of the variable is [0..65535] doesn't mean that
> 'unsigned int' isn't the correct type.
> 
> > 
> > Acked-by: Eugenio Pérez <eperezma@...hat.com>
> > Reviewed-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > Signed-off-by: Jason Wang <jasowang@...hat.com>



I don't like this because it is inconsistent with virtqueue_poll.


If you are going to change this, change virtqueue_enable_cb_prepare_split
too.


But really there's no point.


> > ---
> >  drivers/virtio/virtio_ring.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index 58c03a8aab85..4679a027dc53 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -806,7 +806,7 @@ static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
> >  }
> >  
> >  static bool virtqueue_poll_split(const struct vring_virtqueue *vq,
> > -				 unsigned int last_used_idx)
> > +				 u16 last_used_idx)
> >  {
> >  	return (u16)last_used_idx != virtio16_to_cpu(vq->vq.vdev,
> 
> You can't want that (u16) cast now, I doubt it was ever needed.
> Note that the compiler promotes the value to 'signed int',
> so the LHS of the comparison is actually (int)(u16)last_used_idx.
> 
> 	David

It is not needed because the value is from
virtqueue_enable_cb_prepare_split:

static unsigned int virtqueue_enable_cb_prepare_split(struct virtqueue *_vq)
{
        struct vring_virtqueue *vq = to_vvq(_vq);
        u16 last_used_idx;

        START_USE(vq);

        /* We optimistically turn back on interrupts, then check if there was
         * more to do. */
        /* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
         * either clear the flags bit or point the event index at the next
         * entry. Always do both to keep code simple. */
        if (vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
                vq->split.avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
                if (!vq->event)
                        vq->split.vring.avail->flags =
                                cpu_to_virtio16(_vq->vdev,
                                                vq->split.avail_flags_shadow);
        }
        vring_used_event(&vq->split.vring) = cpu_to_virtio16(_vq->vdev,
                        last_used_idx = vq->last_used_idx);
        END_USE(vq);
        return last_used_idx;
}






> >  			vq->split.vring.used->idx);