linux-kernel - Re: [PATCH v6 6/8] virtio_vdpa: setup correct vq size with callbacks get_vq_num

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Date:   Tue, 26 Oct 2021 12:46:30 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Wu Zongyong <wuzongyong@...ux.alibaba.com>
Cc:     virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        mst <mst@...hat.com>, wei.yang1@...ux.alibaba.com
Subject: Re: [PATCH v6 6/8] virtio_vdpa: setup correct vq size with callbacks get_vq_num_{max,min}

On Mon, Oct 25, 2021 at 2:25 PM Wu Zongyong
<wuzongyong@...ux.alibaba.com> wrote:
>
> On Mon, Oct 25, 2021 at 12:45:44PM +0800, Jason Wang wrote:
> >
> > 在 2021/10/25 上午10:44, Wu Zongyong 写道:
> > > On Mon, Oct 25, 2021 at 10:22:30AM +0800, Jason Wang wrote:
> > > > On Fri, Oct 22, 2021 at 10:45 AM Wu Zongyong
> > > > <wuzongyong@...ux.alibaba.com> wrote:
> > > > > For the devices which implement the get_vq_num_min callback, the driver
> > > > > should not negotiate with virtqueue size with the backend vdpa device if
> > > > > the value returned by get_vq_num_min equals to the value returned by
> > > > > get_vq_num_max.
> > > > > This is useful for vdpa devices based on legacy virtio specfication.
> > > > >
> > > > > Signed-off-by: Wu Zongyong <wuzongyong@...ux.alibaba.com>
> > > > > ---
> > > > >   drivers/virtio/virtio_vdpa.c | 21 ++++++++++++++++-----
> > > > >   1 file changed, 16 insertions(+), 5 deletions(-)
> > > > >
> > > > > diff --git a/drivers/virtio/virtio_vdpa.c b/drivers/virtio/virtio_vdpa.c
> > > > > index 72eaef2caeb1..e42ace29daa1 100644
> > > > > --- a/drivers/virtio/virtio_vdpa.c
> > > > > +++ b/drivers/virtio/virtio_vdpa.c
> > > > > @@ -145,7 +145,8 @@ virtio_vdpa_setup_vq(struct virtio_device *vdev, unsigned int index,
> > > > >          /* Assume split virtqueue, switch to packed if necessary */
> > > > >          struct vdpa_vq_state state = {0};
> > > > >          unsigned long flags;
> > > > > -       u32 align, num;
> > > > > +       u32 align, max_num, min_num = 0;
> > > > > +       bool may_reduce_num = true;
> > > > >          int err;
> > > > >
> > > > >          if (!name)
> > > > > @@ -163,22 +164,32 @@ virtio_vdpa_setup_vq(struct virtio_device *vdev, unsigned int index,
> > > > >          if (!info)
> > > > >                  return ERR_PTR(-ENOMEM);
> > > > >
> > > > > -       num = ops->get_vq_num_max(vdpa);
> > > > > -       if (num == 0) {
> > > > > +       max_num = ops->get_vq_num_max(vdpa);
> > > > > +       if (max_num == 0) {
> > > > >                  err = -ENOENT;
> > > > >                  goto error_new_virtqueue;
> > > > >          }
> > > > >
> > > > > +       if (ops->get_vq_num_min)
> > > > > +               min_num = ops->get_vq_num_min(vdpa);
> > > > > +
> > > > > +       may_reduce_num = (max_num == min_num) ? false : true;
> > > > > +
> > > > >          /* Create the vring */
> > > > >          align = ops->get_vq_align(vdpa);
> > > > > -       vq = vring_create_virtqueue(index, num, align, vdev,
> > > > > -                                   true, true, ctx,
> > > > > +       vq = vring_create_virtqueue(index, max_num, align, vdev,
> > > > > +                                   true, may_reduce_num, ctx,
> > > > >                                      virtio_vdpa_notify, callback, name);
> > > > >          if (!vq) {
> > > > >                  err = -ENOMEM;
> > > > >                  goto error_new_virtqueue;
> > > > >          }
> > > > >
> > > > > +       if (virtqueue_get_vring_size(vq) < min_num) {
> > > > > +               err = -EINVAL;
> > > > > +               goto err_vq;
> > > > > +       }
> > > > I wonder under which case can we hit this?
> > > >
> > > > Thanks
> > > If min_vq_num < max_vq_num, may_reduce_num should be true, then it is
> > > possible to allocate a virtqueue with a small size which value is less
> > > than the min_vq_num since we only set the upper bound for virtqueue size
> > > when creating virtqueue.
> > >
> > > Refers to vring_create_virtqueue_split in driver/virtio/virtio_vring.c:
> > >
> > >     for (; num && vring_size(num, vring_align) > PAGE_SIZE; num /= 2) {
> > >             queue = vring_alloc_queue(vdev, vring_size(num, vring_align),
> > >                                       &dma_addr,
> > >                                       GFP_KERNEL|__GFP_NOWARN|__GFP_ZERO);
> > >             if (queue)
> > >                     break;
> > >             if (!may_reduce_num)
> > >                     return NULL;
> > >     }
> >
> >
> > It looks to me it's better to fix this function instead of checking it in
> > the caller?
>
> Or we can simply remove that code since this case only exists in theory, and
> there is no real usecase for now.

(Adding list back)

Somehow, it can't happen if you stick to a 256 as both min and max.

Another question, can ENI support vring size which is less than 256?

Thanks

>
> >
> >
> > >
> > > BTW, I have replied this mail on Nov.18, have you ever received it?
> >
> >
> > For some reason I dont' get that.
> >
> > Thanks
> >
> >
> > > > > +
> > > > >          /* Setup virtqueue callback */
> > > > >          cb.callback = virtio_vdpa_virtqueue_cb;
> > > > >          cb.private = info;
> > > > > --
> > > > > 2.31.1
> > > > >
>