Re: [PATCH 13/19] virtio_ring: introduce virtqueue ops

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Apr 09, 2025 at 12:06:03PM +0800, Jason Wang wrote:
> On Tue, Apr 8, 2025 at 7:37 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Tue, Apr 08, 2025 at 03:02:35PM +0800, Jason Wang wrote:
> > > On Mon, Apr 7, 2025 at 4:20 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> > > >
> > > > On Mon, Mar 24, 2025 at 02:01:21PM +0800, Jason Wang wrote:
> > > > > This patch introduces virtqueue ops which is a set of the callbacks
> > > > > that will be called for different queue layout or features. This would
> > > > > help to avoid branches for split/packed and will ease the future
> > > > > implementation like in order.
> > > > >
> > > > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > > >
> > > >
> > > >
> > > >
> > > > > ---
> > > > >  drivers/virtio/virtio_ring.c | 96 +++++++++++++++++++++++++-----------
> > > > >  1 file changed, 67 insertions(+), 29 deletions(-)
> > > > >
> > > > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > > > > index a2884eae14d9..ce1dc90ee89d 100644
> > > > > --- a/drivers/virtio/virtio_ring.c
> > > > > +++ b/drivers/virtio/virtio_ring.c
> > > > > @@ -159,9 +159,30 @@ struct vring_virtqueue_packed {
> > > > >       size_t event_size_in_bytes;
> > > > >  };
> > > > >
> > > > > +struct vring_virtqueue;
> > > > > +
> > > > > +struct virtqueue_ops {
> > > > > +     int (*add)(struct vring_virtqueue *_vq, struct scatterlist *sgs[],
> > > > > +                unsigned int total_sg, unsigned int out_sgs,
> > > > > +                unsigned int in_sgs, void *data,
> > > > > +                void *ctx, bool premapped, gfp_t gfp);
> > > > > +     void *(*get)(struct vring_virtqueue *vq, unsigned int *len, void **ctx);
> > > > > +     bool (*kick_prepare)(struct vring_virtqueue *vq);
> > > > > +     void (*disable_cb)(struct vring_virtqueue *vq);
> > > > > +     bool (*enable_cb_delayed)(struct vring_virtqueue *vq);
> > > > > +     unsigned int (*enable_cb_prepare)(struct vring_virtqueue *vq);
> > > > > +     bool (*poll)(const struct vring_virtqueue *vq, u16 last_used_idx);
> > > > > +     void *(*detach_unused_buf)(struct vring_virtqueue *vq);
> > > > > +     bool (*more_used)(const struct vring_virtqueue *vq);
> > > > > +     int (*resize)(struct vring_virtqueue *vq, u32 num);
> > > > > +     void (*reset)(struct vring_virtqueue *vq);
> > > > > +};
> > > >
> > > > I like it that it's organized but
> > > > I worry about the overhead of indirect calls here.
> > >
> > > We can switch to use INDIRECT_CALL_X() here
> >
> > If you think it's cleaner.. but INDIRECT_CALL is all chained
> 
> Yes, and it would be problematic as the number of ops increased.
> 
> > while a switch can do a binary search.
> >
> 
> Do you mean a nested switch?

Not sure what is nested. gcc does a decent job of optimizing
switches. You have 4 types of ops:
packed/packed in order/split/split in order

So:

enum {
	VQ_SPLIT,
	VQ_SPLIT_IN_ORDER,
	VQ_PACKED,
	VQ_PACKED_IN_ORDER,
}


I do not see how it is worse?









> >
> > > (but I'm not sure we
> > > should worry about it too much as ndo_ops or qdiscs doesn't use that).
> >
> >
> > And that's why we ended up with xdp, no? the stack's too heavy ...
> >
> > > > How about a switch statement instead?
> > > >
> > > > struct vring_virtqueue {
> > > >         enum vring_virtqueue_ops ops;
> > > >
> > > > }
> > > >
> > > >
> > > > @@ -2248,10 +2303,8 @@ static inline int virtqueue_add(struct virtqueue *_vq,
> > > >  {
> > > >       struct vring_virtqueue *vq = to_vvq(_vq);
> > > >
> > > >         switch (vq->ops) {
> > > >          VQ_PACKED:
> > > >          VQ_SPLIT:
> > > >          VQ_IN_ORDER:
> > > >         }
> > > >
> > > >
> > > > }
> > > >
> > > >
> > > > What do you think?
> > >
> > > Actually, the matrix will be 2*2:
> > >
> > > PACKED, SPLIT, PACKED_IN_ORDER, SPLIT_IN_ORDER
> >
> > Confused. Same amount of enums as ops structures in your approach, no?
> 
> I meant in this series, we will have 4 ops not 3.
> 
> >
> >
> > > And will be doubled if a new layout is implemented.
> > >
> > > If we open them such a switch will spread in a lot of places in the code.
> > >
> > > Thanks
> 
> Thanks
> 
> > >
> > > >
> > > >
> > > >
> > > > > +
> > > > >  struct vring_virtqueue {
> > > > >       struct virtqueue vq;
> > > > >
> > > > > +     struct virtqueue_ops *ops;
> > > > > +
> > > > >       /* Is this a packed ring? */
> > > > >       bool packed_ring;
> > > > >
> > > > > @@ -1116,6 +1137,8 @@ static int vring_alloc_queue_split(struct vring_virtqueue_split *vring_split,
> > > > >       return 0;
> > > > >  }
> > > > >
> > > > > +struct virtqueue_ops split_ops;
> > > > > +
> > > > >  static struct virtqueue *__vring_new_virtqueue_split(unsigned int index,
> > > > >                                              struct vring_virtqueue_split *vring_split,
> > > > >                                              struct virtio_device *vdev,
> > > > > @@ -1134,6 +1157,7 @@ static struct virtqueue *__vring_new_virtqueue_split(unsigned int index,
> > > > >               return NULL;
> > > > >
> > > > >       vq->packed_ring = false;
> > > > > +     vq->ops = &split_ops;
> > > > >       vq->vq.callback = callback;
> > > > >       vq->vq.vdev = vdev;
> > > > >       vq->vq.name = name;
> > > > > @@ -2076,6 +2100,8 @@ static void virtqueue_reset_packed(struct vring_virtqueue *vq)
> > > > >       virtqueue_vring_init_packed(&vq->packed, !!vq->vq.callback);
> > > > >  }
> > > > >
> > > > > +struct virtqueue_ops packed_ops;
> > > > > +
> > > > >  static struct virtqueue *__vring_new_virtqueue_packed(unsigned int index,
> > > > >                                              struct vring_virtqueue_packed *vring_packed,
> > > > >                                              struct virtio_device *vdev,
> > > > > @@ -2107,6 +2133,7 @@ static struct virtqueue *__vring_new_virtqueue_packed(unsigned int index,
> > > > >       vq->broken = false;
> > > > >  #endif
> > > > >       vq->packed_ring = true;
> > > > > +     vq->ops = &packed_ops;
> > > > >       vq->dma_dev = dma_dev;
> > > > >       vq->use_dma_api = vring_use_dma_api(vdev);
> > > > >
> > > > > @@ -2194,6 +2221,34 @@ static int virtqueue_resize_packed(struct vring_virtqueue *vq, u32 num)
> > > > >       return -ENOMEM;
> > > > >  }
> > > > >
> > > > > +struct virtqueue_ops split_ops = {
> > > > > +     .add = virtqueue_add_split,
> > > > > +     .get = virtqueue_get_buf_ctx_split,
> > > > > +     .kick_prepare = virtqueue_kick_prepare_split,
> > > > > +     .disable_cb = virtqueue_disable_cb_split,
> > > > > +     .enable_cb_delayed = virtqueue_enable_cb_delayed_split,
> > > > > +     .enable_cb_prepare = virtqueue_enable_cb_prepare_split,
> > > > > +     .poll = virtqueue_poll_split,
> > > > > +     .detach_unused_buf = virtqueue_detach_unused_buf_split,
> > > > > +     .more_used = more_used_split,
> > > > > +     .resize = virtqueue_resize_split,
> > > > > +     .reset = virtqueue_reset_split,
> > > > > +};
> > > > > +
> > > > > +struct virtqueue_ops packed_ops = {
> > > > > +     .add = virtqueue_add_packed,
> > > > > +     .get = virtqueue_get_buf_ctx_packed,
> > > > > +     .kick_prepare = virtqueue_kick_prepare_packed,
> > > > > +     .disable_cb = virtqueue_disable_cb_packed,
> > > > > +     .enable_cb_delayed = virtqueue_enable_cb_delayed_packed,
> > > > > +     .enable_cb_prepare = virtqueue_enable_cb_prepare_packed,
> > > > > +     .poll = virtqueue_poll_packed,
> > > > > +     .detach_unused_buf = virtqueue_detach_unused_buf_packed,
> > > > > +     .more_used = more_used_packed,
> > > > > +     .resize = virtqueue_resize_packed,
> > > > > +     .reset = virtqueue_reset_packed,
> > > > > +};
> > > > > +
> > > > >  static int virtqueue_disable_and_recycle(struct virtqueue *_vq,
> > > > >                                        void (*recycle)(struct virtqueue *vq, void *buf))
> > > > >  {
> > > > > @@ -2248,10 +2303,8 @@ static inline int virtqueue_add(struct virtqueue *_vq,
> > > > >  {
> > > > >       struct vring_virtqueue *vq = to_vvq(_vq);
> > > > >
> > > > > -     return vq->packed_ring ? virtqueue_add_packed(vq, sgs, total_sg,
> > > > > -                                     out_sgs, in_sgs, data, ctx, premapped, gfp) :
> > > > > -                              virtqueue_add_split(vq, sgs, total_sg,
> > > > > -                                     out_sgs, in_sgs, data, ctx, premapped, gfp);
> > > > > +     return vq->ops->add(vq, sgs, total_sg,
> > > > > +                         out_sgs, in_sgs, data, ctx, premapped, gfp);
> > > > >  }
> > > > >
> > > > >  /**
> > > > > @@ -2437,8 +2490,7 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
> > > > >  {
> > > > >       struct vring_virtqueue *vq = to_vvq(_vq);
> > > > >
> > > > > -     return vq->packed_ring ? virtqueue_kick_prepare_packed(vq) :
> > > > > -                              virtqueue_kick_prepare_split(vq);
> > > > > +     return vq->ops->kick_prepare(vq);
> > > > >  }
> > > > >  EXPORT_SYMBOL_GPL(virtqueue_kick_prepare);
> > > > >
> > > > > @@ -2508,8 +2560,7 @@ void *virtqueue_get_buf_ctx(struct virtqueue *_vq, unsigned int *len,
> > > > >  {
> > > > >       struct vring_virtqueue *vq = to_vvq(_vq);
> > > > >
> > > > > -     return vq->packed_ring ? virtqueue_get_buf_ctx_packed(vq, len, ctx) :
> > > > > -                              virtqueue_get_buf_ctx_split(vq, len, ctx);
> > > > > +     return vq->ops->get(vq, len, ctx);
> > > > >  }
> > > > >  EXPORT_SYMBOL_GPL(virtqueue_get_buf_ctx);
> > > > >
> > > > > @@ -2531,10 +2582,7 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
> > > > >  {
> > > > >       struct vring_virtqueue *vq = to_vvq(_vq);
> > > > >
> > > > > -     if (vq->packed_ring)
> > > > > -             virtqueue_disable_cb_packed(vq);
> > > > > -     else
> > > > > -             virtqueue_disable_cb_split(vq);
> > > > > +     return vq->ops->disable_cb(vq);
> > > > >  }
> > > > >  EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
> > > > >
> > > > > @@ -2557,8 +2605,7 @@ unsigned int virtqueue_enable_cb_prepare(struct virtqueue *_vq)
> > > > >       if (vq->event_triggered)
> > > > >               vq->event_triggered = false;
> > > > >
> > > > > -     return vq->packed_ring ? virtqueue_enable_cb_prepare_packed(vq) :
> > > > > -                              virtqueue_enable_cb_prepare_split(vq);
> > > > > +     return vq->ops->enable_cb_prepare(vq);
> > > > >  }
> > > > >  EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
> > > > >
> > > > > @@ -2579,8 +2626,7 @@ bool virtqueue_poll(struct virtqueue *_vq, unsigned int last_used_idx)
> > > > >               return false;
> > > > >
> > > > >       virtio_mb(vq->weak_barriers);
> > > > > -     return vq->packed_ring ? virtqueue_poll_packed(vq, last_used_idx) :
> > > > > -                              virtqueue_poll_split(vq, last_used_idx);
> > > > > +     return vq->ops->poll(vq, last_used_idx);
> > > > >  }
> > > > >  EXPORT_SYMBOL_GPL(virtqueue_poll);
> > > > >
> > > > > @@ -2623,8 +2669,7 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
> > > > >       if (vq->event_triggered)
> > > > >               vq->event_triggered = false;
> > > > >
> > > > > -     return vq->packed_ring ? virtqueue_enable_cb_delayed_packed(vq) :
> > > > > -                              virtqueue_enable_cb_delayed_split(vq);
> > > > > +     return vq->ops->enable_cb_delayed(vq);
> > > > >  }
> > > > >  EXPORT_SYMBOL_GPL(virtqueue_enable_cb_delayed);
> > > > >
> > > > > @@ -2640,14 +2685,13 @@ void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
> > > > >  {
> > > > >       struct vring_virtqueue *vq = to_vvq(_vq);
> > > > >
> > > > > -     return vq->packed_ring ? virtqueue_detach_unused_buf_packed(vq) :
> > > > > -                              virtqueue_detach_unused_buf_split(vq);
> > > > > +     return vq->ops->detach_unused_buf(vq);
> > > > >  }
> > > > >  EXPORT_SYMBOL_GPL(virtqueue_detach_unused_buf);
> > > > >
> > > > >  static inline bool more_used(const struct vring_virtqueue *vq)
> > > > >  {
> > > > > -     return vq->packed_ring ? more_used_packed(vq) : more_used_split(vq);
> > > > > +     return vq->ops->more_used(vq);
> > > > >  }
> > > > >
> > > > >  /**
> > > > > @@ -2785,10 +2829,7 @@ int virtqueue_resize(struct virtqueue *_vq, u32 num,
> > > > >       if (recycle_done)
> > > > >               recycle_done(_vq);
> > > > >
> > > > > -     if (vq->packed_ring)
> > > > > -             err = virtqueue_resize_packed(vq, num);
> > > > > -     else
> > > > > -             err = virtqueue_resize_split(vq, num);
> > > > > +     err = vq->ops->resize(vq, num);
> > > > >
> > > > >       return virtqueue_enable_after_reset(_vq);
> > > > >  }
> > > > > @@ -2822,10 +2863,7 @@ int virtqueue_reset(struct virtqueue *_vq,
> > > > >       if (recycle_done)
> > > > >               recycle_done(_vq);
> > > > >
> > > > > -     if (vq->packed_ring)
> > > > > -             virtqueue_reset_packed(vq);
> > > > > -     else
> > > > > -             virtqueue_reset_split(vq);
> > > > > +     vq->ops->reset(vq);
> > > > >
> > > > >       return virtqueue_enable_after_reset(_vq);
> > > > >  }
> > > > > --
> > > > > 2.42.0
> > > >
> >