Date:   Fri, 11 Feb 2022 15:36:15 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 09/14] virtio: queue_reset: struct virtio_config_ops
 add callbacks for queue_reset

On Fri, Feb 11, 2022 at 3:13 PM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> wrote:
>
> On Fri, 11 Feb 2022 14:49:03 +0800, Jason Wang <jasowang@...hat.com> wrote:
> >
> > 在 2022/2/9 下午8:28, Xuan Zhuo 写道:
> > > Performing reset on a queue is divided into four steps:
> > >
> > > 1. reset_vq: reset one vq
> > > 2. recycle the buffer from vq by virtqueue_detach_unused_buf()
> > > 3. release the ring of the vq by vring_release_virtqueue()
> > > 4. enable_reset_vq: re-enable the reset queue
> > >
> > > So add two callbacks reset_vq, enable_reset_vq to struct
> > > virtio_config_ops.
> > >
> > > Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> > > ---
> > >   include/linux/virtio_config.h | 13 +++++++++++++
> > >   1 file changed, 13 insertions(+)
> > >
> > > diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
> > > index 4d107ad31149..0d01a64f2576 100644
> > > --- a/include/linux/virtio_config.h
> > > +++ b/include/linux/virtio_config.h
> > > @@ -74,6 +74,17 @@ struct virtio_shm_region {
> > >    * @set_vq_affinity: set the affinity for a virtqueue (optional).
> > >    * @get_vq_affinity: get the affinity for a virtqueue (optional).
> > >    * @get_shm_region: get a shared memory region based on the index.
> > > + * @reset_vq: reset a queue individually
> >
> >
> > This needs to be marked as optional I think.
>
> OK.
>
> >
> >
> > > + * vq: the virtqueue
> > > + * Returns 0 on success or error status
> >
> >
> > It looks to me the caller should also guarantee that the vring is not
> > accsed by any functions e.g NAPI.
>
> OK.
>
> >
> >
> > > + * After successfully calling this, be sure to call
> > > + * virtqueue_detach_unused_buf() to recycle the buffer in the ring, and
> > > + * then call vring_release_virtqueue() to release the vq ring.
> > > + * @enable_reset_vq: enable a reset queue
> > > + * vq: the virtqueue
> > > + * ring_num: specify ring num for the vq to be re-enabled. 0 means use the
> > > + *           default value. MUST be a power of 2.
> >
> >
> > Note that we don't have power of 2 requirement for packed virtqueue.
>
>
> So the following check here does not seem reasonable. (virtio_pci_modern.c)

Rethink about this, looks like I was wrong.

Though packed virtqueue doesn't mandate power of 2, the actual queue
format should be invisible to a driver.

So forcing power of 2 is correct (since it was required by split virtqueue)

>
> static struct virtqueue *setup_vq(struct virtio_pci_device *vp_dev,
>                                   struct virtio_pci_vq_info *info,
>                                   unsigned index,
>                                   void (*callback)(struct virtqueue *vq),
>                                   const char *name,
>                                   bool ctx,
>                                   u16 msix_vec)
> {
>
>         struct virtio_pci_modern_device *mdev = &vp_dev->mdev;
>         struct virtqueue *vq;
>         u16 num;
>         int err;
>
>         if (index >= vp_modern_get_num_queues(mdev))
>                 return ERR_PTR(-ENOENT);
>
>         /* Check if queue is either not available or already active. */
>         num = vp_modern_get_queue_size(mdev, index);
>         if (!num || vp_modern_get_queue_enable(mdev, index))
>                 return ERR_PTR(-ENOENT);
>
>         if (num & (num - 1)) {
>                 dev_warn(&vp_dev->pci_dev->dev, "bad queue size %u", num);
>                 return ERR_PTR(-EINVAL);
>         }
>         ^^^^^^^^^^^^^^^^^^^^^^^
>
> >
> > And I wonder if it's cleaner to have a find_vq() ops instead to dealing
> > with the re-allocation and possible size change, or have a dedicated
> > helper to set vring size so driver can do.
> >
> > reset_vq()
> >
> > virtqueue_set_vring_size()
> >
> > enable_reset_vq()
>
> I like to add a dedicated helper.
>
> >
> >
> > > + * Returns 0 on success or error status
> > >    */
> > >   typedef void vq_callback_t(struct virtqueue *);
> > >   struct virtio_config_ops {
> > > @@ -100,6 +111,8 @@ struct virtio_config_ops {
> > >                     int index);
> > >     bool (*get_shm_region)(struct virtio_device *vdev,
> > >                            struct virtio_shm_region *region, u8 id);
> > > +   int (*reset_vq)(struct virtqueue *vq);
> > > +   int (*enable_reset_vq)(struct virtqueue *vq, u16 ring_num);
> >
> >
> > Note that the current implement is best-effort, so it's not guarantee
> > that we can have a vring with ring_num, we may get less under memory
> > pressure or even fail. We probably need to have a pamater to mandate the
> > ring_num otherwise user may surprise to see a decreased size of the ring
> > when a increasing is actually requested.
>
> 1. We can add a helper to specify max ring num.
> 2. Or after specifying ring num, in case of failure, return directly.
>
> I prefer #1

That's fine.

Thanks

>
> Thanks.
>
> >
> > Thanks
> >
> >
> > >   };
> > >
> > >   /* If driver didn't advertise the feature, it will never appear. */
> >
>

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