Re: [PATCH v7 09/26] virtio_ring: split: implement virtqueue_reset_vring_split()

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Thu, 10 Mar 2022 02:00:39 -0500, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> On Tue, Mar 08, 2022 at 08:35:01PM +0800, Xuan Zhuo wrote:
> > virtio ring supports reset.
> >
> > Queue reset is divided into several stages.
> >
> > 1. notify device queue reset
> > 2. vring release
> > 3. attach new vring
> > 4. notify device queue re-enable
> >
> > After the first step is completed, the vring reset operation can be
> > performed. If the newly set vring num does not change, then just reset
> > the vq related value.
> >
> > Otherwise, the vring will be released and the vring will be reallocated.
> > And the vring will be attached to the vq. If this process fails, the
> > function will exit, and the state of the vq will be the vring release
> > state. You can call this function again to reallocate the vring.
> >
> > In addition, vring_align, may_reduce_num are necessary for reallocating
> > vring, so they are retained when creating vq.
> >
> > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > ---
> >  drivers/virtio/virtio_ring.c | 69 ++++++++++++++++++++++++++++++++++++
> >  1 file changed, 69 insertions(+)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index e0422c04c903..148fb1fd3d5a 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -158,6 +158,12 @@ struct vring_virtqueue {
> >  			/* DMA address and size information */
> >  			dma_addr_t queue_dma_addr;
> >  			size_t queue_size_in_bytes;
> > +
> > +			/* The parameters for creating vrings are reserved for
> > +			 * creating new vrings when enabling reset queue.
> > +			 */
> > +			u32 vring_align;
> > +			bool may_reduce_num;
> >  		} split;
> >
> >  		/* Available for packed ring */
> > @@ -217,6 +223,12 @@ struct vring_virtqueue {
> >  #endif
> >  };
> >
> > +static void vring_free(struct virtqueue *vq);
> > +static void __vring_virtqueue_init_split(struct vring_virtqueue *vq,
> > +					 struct virtio_device *vdev);
> > +static int __vring_virtqueue_attach_split(struct vring_virtqueue *vq,
> > +					  struct virtio_device *vdev,
> > +					  struct vring vring);
> >
> >  /*
> >   * Helpers.
> > @@ -1012,6 +1024,8 @@ static struct virtqueue *vring_create_virtqueue_split(
> >  		return NULL;
> >  	}
> >
> > +	to_vvq(vq)->split.vring_align = vring_align;
> > +	to_vvq(vq)->split.may_reduce_num = may_reduce_num;
> >  	to_vvq(vq)->split.queue_dma_addr = vring.dma_addr;
> >  	to_vvq(vq)->split.queue_size_in_bytes = vring.queue_size_in_bytes;
> >  	to_vvq(vq)->we_own_ring = true;
> > @@ -1019,6 +1033,59 @@ static struct virtqueue *vring_create_virtqueue_split(
> >  	return vq;
> >  }
> >
> > +static int virtqueue_reset_vring_split(struct virtqueue *_vq, u32 num)
> > +{
> > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > +	struct virtio_device *vdev = _vq->vdev;
> > +	struct vring_split vring;
> > +	int err;
> > +
> > +	if (num > _vq->num_max)
> > +		return -E2BIG;
> > +
> > +	switch (vq->vq.reset) {
> > +	case VIRTIO_VQ_RESET_STEP_NONE:
> > +		return -ENOENT;
> > +
> > +	case VIRTIO_VQ_RESET_STEP_VRING_ATTACH:
> > +	case VIRTIO_VQ_RESET_STEP_DEVICE:
> > +		if (vq->split.vring.num == num || !num)
> > +			break;
> > +
> > +		vring_free(_vq);
> > +
> > +		fallthrough;
> > +
> > +	case VIRTIO_VQ_RESET_STEP_VRING_RELEASE:
> > +		if (!num)
> > +			num = vq->split.vring.num;
> > +
> > +		err = vring_create_vring_split(&vring, vdev,
> > +					       vq->split.vring_align,
> > +					       vq->weak_barriers,
> > +					       vq->split.may_reduce_num, num);
> > +		if (err)
> > +			return -ENOMEM;
> > +
> > +		err = __vring_virtqueue_attach_split(vq, vdev, vring.vring);
> > +		if (err) {
> > +			vring_free_queue(vdev, vring.queue_size_in_bytes,
> > +					 vring.queue,
> > +					 vring.dma_addr);
> > +			return -ENOMEM;
> > +		}
> > +
> > +		vq->split.queue_dma_addr = vring.dma_addr;
> > +		vq->split.queue_size_in_bytes = vring.queue_size_in_bytes;
> > +	}
> > +
> > +	__vring_virtqueue_init_split(vq, vdev);
> > +	vq->we_own_ring = true;
> > +	vq->vq.reset = VIRTIO_VQ_RESET_STEP_VRING_ATTACH;
> > +
> > +	return 0;
> > +}
> > +
>
> I kind of dislike this state machine.
>
> Hacks like special-casing num = 0 to mean "reset" are especially
> confusing.

I'm removing it. I'll say in the function description that this function is
currently only called when vq has been reset. I'm no longer checking it based on
state.

>
> And as Jason points out, when we want a resize then yes this currently
> implies reset but that is an implementation detail.
>
> There should be a way to just make these cases separate functions
> and then use them to compose consistent external APIs.

Yes, virtqueue_resize_split() is fine for ethtool -G.

But in the case of AF_XDP, just execute reset to free the buffer. The name
virtqueue_reset_vring_split() I think can cover both cases. Or we use two apis
to handle both scenarios?

Or can anyone think of a better name. ^_^

Thanks.

>
> If we additionally want to track state for debugging then bool flags
> seem more appropriate for this, though from experience that is
> not always worth the extra code.
>
>
>
> >  /*
> >   * Packed ring specific functions - *_packed().
> > @@ -2317,6 +2384,8 @@ static int __vring_virtqueue_attach_split(struct vring_virtqueue *vq,
> >  static void __vring_virtqueue_init_split(struct vring_virtqueue *vq,
> >  					 struct virtio_device *vdev)
> >  {
> > +	vq->vq.reset = VIRTIO_VQ_RESET_STEP_NONE;
> > +
> >  	vq->packed_ring = false;
> >  	vq->we_own_ring = false;
> >  	vq->broken = false;
> > --
> > 2.31.0
>

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