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

Xuan Zhuo
2022-03-10T12:35:55Z

On Thu, 10 Mar 2022 07:17:09 -0500, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> On Thu, Mar 10, 2022 at 04:14:16PM +0800, Xuan Zhuo wrote:
> > On Thu, 10 Mar 2022 03:07:22 -0500, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > > On Thu, Mar 10, 2022 at 03:17:03PM +0800, Xuan Zhuo wrote:
> > > > 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.
> > >
> > >
> > > I'd say resize should be called resize and reset should be called reset.
> >
> >
> > OK, I'll change it to resize here.
> >
> > But I want to know that when I implement virtio-net to support AF_XDP, its
> > requirement is to release all submitted buffers. Then should I add a new api
> > such as virtqueue_reset_vring()?
>
> Sounds like a reasonable name.
>
> > >
> > > The big issue is a sane API for resize. Ideally it would resubmit
> > > buffers which did not get used. Question is what to do
> > > about buffers which don't fit (if ring has been downsized)?
> > > Maybe a callback that will handle them?
> > > And then what? Queue them up and readd later? Drop?
> > > If we drop we should drop from the head not the tail ...
> >
> > It's a good idea, let's implement it later.
> >
> > Thanks.
>
> Well ... not sure how you are going to support resize
> if you don't know what to do with buffers that were
> in the ring.

The current solution is to call virtqueue_detach_unused_buf() to release buffers
before resize ring.

Thanks.