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

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Message-ID: <CACGkMEuSSGNFMiwcD5UBvpQcUxtLmA-2PiT3AD_Sf3YSzndjMQ@mail.gmail.com>
Date: Thu, 9 Oct 2025 13:55:51 +0800
From: Jason Wang <jasowang@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: David Laight <david.laight.linux@...il.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 Mon, Sep 29, 2025 at 4:22 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> 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);
>

I will drop this in the next version.

Thanks